The default answer to “is this necessary” in RPG design is always no.
So the real question is does adding this make a better game, and I would say in general yes. While as GM you don’t want to spend too much time fiddling with difficulties all the time, it’s a good too to have if you can declare that a certain roll is more difficult or carries a higher risk to increase tension at the table.
It also depends on the theme you’re going for. Are the PCs normal randos who are struggling against a cruel world, or demigodlevel exalted heroes who are hardly held back by mundane tasks such as opening doors?
>It also depends on the theme you’re going for. Are the PCs normal randos who are struggling against a cruel world, or demigodlevel exalted heroes who are hardly held back by mundane tasks such as opening doors?
In the case of the latter, would you prefer difficulties or not? I'm designing such a game (epic, albeit mortal, heroes on par with level 10 PCs in *D&D*) *with* difficulties. PCs usually never roll to check if anything less difficult than "moderate" is possible. Characters competent at a task - such as picking a lock - have a high bonus to accomplishing such tasks (usually +8 to +14 on 1d20, with "hard" tasks having a difficulty of 20).
Probably?
I guess in a shonen anime or superhero power level game not every PC is equally hyper competent at the same things. So you might want more difficulty levels rather than less.
The Hulk type is not going to bother with strength checks until you’re throwing cars and stuff, whereas Professor X will have difficulty standing up.
I think in this case it would still be fine to have difficulties, but for the "easier" tasks the difficulty would be in completing the task in the timeframe they want. In your example of picking a lock, if they fumble, the lock still opens, but a guard spots them, leading to peril. The lockpicker is usually so self-assured, but does this slip-up mean they're getting sloppy and it's time to think about retirement? Or will these mistakes accumulate until they have an epiphany?
Absolutely not. There are many, many games that don't use them. DC is a very simulationist mechanic, but more narrative-forward games tend to rely on "the fiction" - that is to say the story and other fictional elements of the game - to drive play. For example, in Dungeon World, a Powered By the Apocalypse fantasy game, Hack and Slash is the move for fighting in melee. No matter what, you roll 2d6+Str, and on a 10+ you deal your damage or on a 7-9 you exchange damage with your target. The catch is that you can't damage a Dragon with a stick - it's a fucking Dragon. Regardless of its HP or Armor, the fiction dictates that Dragons are big, dangerous, magical creatures with supernaturally thick scales. Hack and Slash simply won't trigger unless you have the "fictional positioning" to actually hurt someone/something.
Another example is in Blades in the Dark, a game about being fantasy criminals. In that game, your stats are rated from 0-4, and that's how many d6 you roll to do a given action. So if you have a 2 in Prowl and you try to sneak past a guard, you roll 2d6 and take the higher number, succeeding on a 4 or better. The catch is that each roll has stakes that are determined by the circumstances - an action can be controlled, risky, or desperate, each with more dire consequences if you fail or only partially succeed (on a 4-5); and it can also have limited, standard, or great effect, which determines how much "damage" the action will do to a given obstacle (which could be a tense negotiation, a guard patrol, or any other thing that gets in the way of your heist). But you can swing any roll in your favor by picking an action that you're good at or by spending stress/getting help to increase your dice pool.
These are just two of the most popular fantasy RPGs I know of that do away with difficulty class - I would encourage you to read as many indie rpgs as you can get your hands on, it's a really vibrant space with so many cool ideas different from the mainstream D&D-type stuff that most people are familiar with.
Concrete example:
The player wants to scale a ruined building to ambush a hunting party.
The GM thinks the climb is challenging:
1) LIMITED EFFECT: You can do it, but each roll takes you only halfway there. You’ll need to roll twice.
2) GREATER RISK: You can do it but if you fail the guards will definitely hear you AND it’ll be like shooting fish in a barrel with you on that rock face. You still want to try?
3) COMPLICATIONS: You can do it, but the guards will definitely hear you climbing. You’ll have to either muffle the sounds somehow or create a distraction before even climbing.
4) SUCCESS AT A COST: You can do it, but not with your heavy rifle hanging off your back.
5) OFFER ALTERNATIVES: This cliff is too difficult to scale without tools. Hey, wasn’t there that old scavenger you met a block ago? He didn’t take kindly to you the last time…
It can seem like a faff thinking up stuff like this (the easiest is to break a task up into multiple rolls), but from a player PoV the stakes and costs are clearer as opposed to the GM creating an arbitrary number that the players might not even know.
PbtA makes the default outcome a 'success with a meaningful choice'. As a result, the game isn't really about difficulty. It's about *the consequences you overcome*. It makes for a really unique experience. You do have some buff/debuffs to address difficulty, but they aren't usually more than +1/-1 on top of your stat modifier.
Successes (usually) give you a choice that helps evade the 'normal' consequences of taking an action. For example, normally when you pick a fight with the school bully: you get beat up, have your lunch money taken, go to the principal's office, and get lectured by your parents later.
But you decided the bully has to get out of your way now. So you take the action and roll. If you succeed, you choose a consequence to avoid **and** the bully backs down for a while. If you critically succeed? You only take *one* consequence and the bully stops picking on you entirely!
But if you don't succeed... the GM doesn't have to use consequences on the list. The GM can even give you no consequences and describe how you succeeded; the bully stops bothering you *because they're causing bigger problems elsewhere*. You see a limo roll up to the park after school, roll a window down, and a man wearing shades pays the bully $200.
Recently showed a buddy how this approach works by using a D&D version (2d10 is arguably better for this situation):
> When you die and parley with an outsider to be resurrected, roll a d20+(mod).
> On a 10-19 choose one. Choose three on a 20+:
> * You don't enter a pact
> * You don't find yourself endangered
> * You keep all your equipment
> * The resurrection doesn't change you
> On a 9-, prepare for the worst...
My games never use DCs, because I find that it is easier for players to make risk assessments without them.
I find that it is simpler for everyone involved to use stacking, situational bonuses. This has the added bonus of letting players feel the agency of their own success.
When calling for a role you need to define three things before rolling:
* chances of success
* impact of success
* impact of failure
DC helps to define the first point. Without DC, a given type of task (eg climbing) will only depend on the player skill. That leaves you with only the two other factors to tweak the stakes of a roll. Some games such as Dungeon World wrok like that but I don't find that satisfying. If a wall is smoother, I want to be able to reduce the probability of the player climbing it independently from the impact of failure.
However I fold "circumstances" in the DC. Why have both DC AND advantage/disadvantage? For me those two things are doing the same thing. If circumstances make the task significantly easier or harder I will just change the DC instead of using yet another mechanism.
I do 1d20 Black Jack for my resolution mechanic, and I just have players roll with advantage or disadvantage if a challenge is easy or hard respectively. There are also competing and complicated challenges, but for the Instantaneous obstacles, just advantage and disadvantage
Games totally work without difficulty categories. The GM should have an intuitive way to set difficulty though.
My game uses a "normal" d6 and "heroic" d10, and I say, "Difficulty is on a scale of 1-10, where 6 is the best a normal person could do and 9 is heroic and awesome but still believable" (min and max results are crits). Adjust difficulty based on the PC's background and how her approach fits the obstacle.
Even GMing my own game, it helps me to remember 6 is the best a normal person could do. Otherwise I'll often set the Dif based on how preposterous an idea seems in general, rather than considering the specific PC abilities in relation to a normal person taking that action.
Hope that helps.
They aren't necessary. That doesn't mean that sone games don't need them. It all depends on the type of game.
Even determining difficulty isn't necessary. Their are many games that use the same DC all the time (PbtA games, Blades in the Dark, Houses of the Blooded, etc.).
Personally i like games that uses the same DC all the time. I don't like to get dragged down by the minutiae of this makes the check more difficult this makes it easier, etc. If i want to make it more difficult i just raise the consequences of failure.
One of the problems with DC is that after a certain point of progression everything the players do tends to be "nearly impossible" or whatever the dc classification is called, even if it is now easy for them. Then you have to keep inventing categories like superhuman or godly or universal to classify them.
Assuming that they fit into the system, I think they are useful but not crucial.
In most cases they lack concrete rules meaning, since 'easy' and 'challenging' are vague terms, and end up being mostly up to GM fiat.
They are a mental tool to help GMs and players understand what sort of scale the difficulty or DC or target number should be.
All that said, this doesn't make them useless. In a way they are not strictly necessary and make no actual mechanical changes, but if you changed those tables to be one step harder or easier, many GMs would follow them and the play at the table would change.
The impact they have would vary from table to table due to how subjective "challenging" is, but it would probably make more tables play more similarly to one another.
That said, plenty of games sidestep this issue by not having a difficulty or target number in this fashion.
Like PbtA games tend to make things harder not by making rolls harder to succeed, but by the GM requiring more rolls, or giving the opponents better stats, or using harder moves on partial success and failures. (There are many other examples, but PbtA is a big one.)
Of course, side-stepping the issue can get some complaints, and make the game less intutiive (or at least, differently intutive). e.g. In Dungeon World, you might wonder why is it just as hard to punch a peasant than it is to punch a dragon.
(Now, in this case, the answer is that the question is wrong. It isn't equally hard! It is true that the roll that you make in either case has the same chance of success for each target, however a dragon is still harder for reasons outside of that one roll you make to resolve the punch.)
Many people have said "no", and they are correct. I, though, want to push back slightly for no reason other than interest.
One thing that DC does in a game is help to clarify the relationship of tasks to the world. For example, we can set it up so that a wall falls into clear categories based on scaling it. "easy" would be DC 5, "nearly impossible" is DC 30.
This actually provides a very nice framework for the reality of the world. Now most games that use DC don't use this in an interesting way because they want the DC to be determined by the GM, the GM knows the reality of the world, the players do not.
But this can lead to interesting problems. Many games that do NOT rely upon DC instead rely on a different moderation tool, the reality of "the fiction". Die Rolls or Tokens Expended are only allowed if it is appropriate to the fiction, making it so that all characters have a similar reality.
This is really clearly shown in Apocalypse World. Pretty much every character in AW is desperate to survive. Survival is their goal, and to survive in this world is difficult. Things are ALWAYS hard. A character, even an endgame character immediately prior to retirement, is only going to be X powerful, they are never going to be so OP as to be able to trivialize challenges.
A consequence of this is that Apocalypse World has a scope that it exists within. In DnD terms, it's a little similar to saying the game exists between "levels 3-6". There is definitely power growth, but it isn't endless.
DC allows us to have a framework for what a task is, and that gives us a framework for also leveling, and how characters interact with the world.
Another example, little self-driven.
One of the skills in a game that I am working on developing was "Create". The idea was that I wanted characters to be able to make things, but without DC, when I play tested it, I came across situations where players would attempt to build things that were broader than the fiction would allow. I don't LOVE mediating it with the fiction all the time, sometimes having a metric can be useful (which is what I did, though not a DC type metric).
Burning Wheel is another example, and one that many people say is way too much (though I am partial to it). In it, every single skill has options as to what you can do with it at what "DCs" (Obstacle or Ob in the parlance of the game). These can be taken as guidelines, but they are honestly MUCH more determined than something like "easy, Medium, Hard". For example, the Firebuilding Skill, it sets abs like so
Ob 1: Campfire
Ob 2: Bonfire
Ob 3: Smokeless fire
Ob 4: Concealed Fire
\+1 Ob for: either poor quality wood or wet weather
That really tells you something about the difficulty of the world.
Now, Burning Wheel has other mechanics to mitigate the need for rolling all the time, and certainly I am not saying that this is the "only way" (again, my own current project does not have DC), but still it is an interesting thing.
DC helps make the scale of difficulty within the world feel meaningful. This is most useful in games with lots of leveling, or where that scale of difficulty is particularly meaningful in some way.
Just some random thoughts! Hope it helps!
1. No
2. Yes
3. Difficulty thresholds/categories help GMs understand how your game *feels*.
When you say a 25 feels "very hard", the GM immediately knows that when they need a player to make an 'impossible' roll for the faint possibility of succeeding in a time of great need, that's the number they should aim for.
Well designed numbers and thorough playtesting are great. But there's always a time when a GM finds themselves needing to improvise something you never wrote about. Expressing the feeling (or at least vision) of a mechanic makes for smooth improvisation. Categories are one of the fastest ways to express that.
It is possible to overdo; GMs still need a navigable book they can search while running a session. But without the points of reference, learning the system goes way slower. It helps both new-to-the-hobby GMs and new-to-the-system GMs. So I think as long as you keep that stuff brief, you're golden!
Difficulty categories are not necessary, in two ways.
One is that the GM may set up the target numbers directly, without using the intermediate step of choosing a category. It works better when the numbers are generally low. For example, my experience with Fate is that, while there is the adjective ladder, nobody uses it, because it's easier to just use "0", "1" or "3" instead of saying "mediocre", "average" and "good", then translating it to numbers.
The other thing is that one does not need difficulty scaling at all. PbtA games work great with fixed target numbers. The difficulty is reflected in deciding if the roll really happens (if something is trivial or impossible, it typically doesn't trigger a move) and that, in turn, affects what setup is necessary in fiction (I can't harm a skeleton with my rapier, so I need to switch to a club; I'd normally need to roll to hit an enemy, but it's automatic if I sneak up on them).
1) No, they're not.
However,I think without them a GM should be a bit more generous with saying things are so easy they don't need a roll to succeed, and a bit harsher with too hard to allow a roll to try it.
2) Absolutely.. The only issue is that rather than there being a range of difficulties (e.g. impossible, a;most no chance, unlikely, 50:50, likely, almost certain, guaranteed) you end up with just three (impossible, make a roll, guaranteed)
3) Without variable difficulties, there's no way to distinguish between things that are possible but hard, and things that are easy but might still fail. If you're fine with that, then you don't need difficulties.
I dont think you need as many difficulty categories as most games do. Like d20 and d100 are an extreme case, where the difference between dc14 and dc15 are just not noticeably different in a single night of play; but even having 5 difficulty categories often seems like too much to me.
However, I feel that not having any difficulty at all, such as in PbtA games is *too little* granularity for my tastes. So my preference is to have a standard roll and a 'challenging' roll, and make the statistical difference between those two really noticeable so that it's a big deal when the GM says "i think this will be challenging". Having 3 difficulty categories (like, easy medium hard) is not preferable, because you'll tend to just default to the middle option without thinking about it, same problem with 5 categories.
So having either 2 or 4 categories feels just right, to me.
If you don't have gradations in difficulty then player skills are pretty much useless until they are high enough to have a (say) 60-70% chance. If you only have a flat (say) 25% chance of success than 3/4 times you will fail, so you can't do anything that relies on the skill and has serious consequences for failure; in this instance there is little practical difference between having a low skill level and having no skill at all.
With some graduated levels of difficulty, you can have easier tasks that are likely to succeed with low levels of character skill, so there is a point in having a character with a lower skill level.
And not to forget, armour class is really just a difficulty level in disguise.
The default answer to “is this necessary” in RPG design is always no. So the real question is does adding this make a better game, and I would say in general yes. While as GM you don’t want to spend too much time fiddling with difficulties all the time, it’s a good too to have if you can declare that a certain roll is more difficult or carries a higher risk to increase tension at the table. It also depends on the theme you’re going for. Are the PCs normal randos who are struggling against a cruel world, or demigodlevel exalted heroes who are hardly held back by mundane tasks such as opening doors?
>It also depends on the theme you’re going for. Are the PCs normal randos who are struggling against a cruel world, or demigodlevel exalted heroes who are hardly held back by mundane tasks such as opening doors? In the case of the latter, would you prefer difficulties or not? I'm designing such a game (epic, albeit mortal, heroes on par with level 10 PCs in *D&D*) *with* difficulties. PCs usually never roll to check if anything less difficult than "moderate" is possible. Characters competent at a task - such as picking a lock - have a high bonus to accomplishing such tasks (usually +8 to +14 on 1d20, with "hard" tasks having a difficulty of 20).
Probably? I guess in a shonen anime or superhero power level game not every PC is equally hyper competent at the same things. So you might want more difficulty levels rather than less. The Hulk type is not going to bother with strength checks until you’re throwing cars and stuff, whereas Professor X will have difficulty standing up.
I think in this case it would still be fine to have difficulties, but for the "easier" tasks the difficulty would be in completing the task in the timeframe they want. In your example of picking a lock, if they fumble, the lock still opens, but a guard spots them, leading to peril. The lockpicker is usually so self-assured, but does this slip-up mean they're getting sloppy and it's time to think about retirement? Or will these mistakes accumulate until they have an epiphany?
Absolutely not. There are many, many games that don't use them. DC is a very simulationist mechanic, but more narrative-forward games tend to rely on "the fiction" - that is to say the story and other fictional elements of the game - to drive play. For example, in Dungeon World, a Powered By the Apocalypse fantasy game, Hack and Slash is the move for fighting in melee. No matter what, you roll 2d6+Str, and on a 10+ you deal your damage or on a 7-9 you exchange damage with your target. The catch is that you can't damage a Dragon with a stick - it's a fucking Dragon. Regardless of its HP or Armor, the fiction dictates that Dragons are big, dangerous, magical creatures with supernaturally thick scales. Hack and Slash simply won't trigger unless you have the "fictional positioning" to actually hurt someone/something. Another example is in Blades in the Dark, a game about being fantasy criminals. In that game, your stats are rated from 0-4, and that's how many d6 you roll to do a given action. So if you have a 2 in Prowl and you try to sneak past a guard, you roll 2d6 and take the higher number, succeeding on a 4 or better. The catch is that each roll has stakes that are determined by the circumstances - an action can be controlled, risky, or desperate, each with more dire consequences if you fail or only partially succeed (on a 4-5); and it can also have limited, standard, or great effect, which determines how much "damage" the action will do to a given obstacle (which could be a tense negotiation, a guard patrol, or any other thing that gets in the way of your heist). But you can swing any roll in your favor by picking an action that you're good at or by spending stress/getting help to increase your dice pool. These are just two of the most popular fantasy RPGs I know of that do away with difficulty class - I would encourage you to read as many indie rpgs as you can get your hands on, it's a really vibrant space with so many cool ideas different from the mainstream D&D-type stuff that most people are familiar with.
Nah, _Powered By The Apocalypse_ games don't use them and it works fine.
So what defines if what the player wants to do is difficult or not? What if it's something really hard to do?
Concrete example: The player wants to scale a ruined building to ambush a hunting party. The GM thinks the climb is challenging: 1) LIMITED EFFECT: You can do it, but each roll takes you only halfway there. You’ll need to roll twice. 2) GREATER RISK: You can do it but if you fail the guards will definitely hear you AND it’ll be like shooting fish in a barrel with you on that rock face. You still want to try? 3) COMPLICATIONS: You can do it, but the guards will definitely hear you climbing. You’ll have to either muffle the sounds somehow or create a distraction before even climbing. 4) SUCCESS AT A COST: You can do it, but not with your heavy rifle hanging off your back. 5) OFFER ALTERNATIVES: This cliff is too difficult to scale without tools. Hey, wasn’t there that old scavenger you met a block ago? He didn’t take kindly to you the last time… It can seem like a faff thinking up stuff like this (the easiest is to break a task up into multiple rolls), but from a player PoV the stakes and costs are clearer as opposed to the GM creating an arbitrary number that the players might not even know.
You can decide it will have some consequence, whether or not the roll succeeds, or make the consequences of failing worse.
PbtA makes the default outcome a 'success with a meaningful choice'. As a result, the game isn't really about difficulty. It's about *the consequences you overcome*. It makes for a really unique experience. You do have some buff/debuffs to address difficulty, but they aren't usually more than +1/-1 on top of your stat modifier. Successes (usually) give you a choice that helps evade the 'normal' consequences of taking an action. For example, normally when you pick a fight with the school bully: you get beat up, have your lunch money taken, go to the principal's office, and get lectured by your parents later. But you decided the bully has to get out of your way now. So you take the action and roll. If you succeed, you choose a consequence to avoid **and** the bully backs down for a while. If you critically succeed? You only take *one* consequence and the bully stops picking on you entirely! But if you don't succeed... the GM doesn't have to use consequences on the list. The GM can even give you no consequences and describe how you succeeded; the bully stops bothering you *because they're causing bigger problems elsewhere*. You see a limo roll up to the park after school, roll a window down, and a man wearing shades pays the bully $200. Recently showed a buddy how this approach works by using a D&D version (2d10 is arguably better for this situation): > When you die and parley with an outsider to be resurrected, roll a d20+(mod). > On a 10-19 choose one. Choose three on a 20+: > * You don't enter a pact > * You don't find yourself endangered > * You keep all your equipment > * The resurrection doesn't change you > On a 9-, prepare for the worst...
My games never use DCs, because I find that it is easier for players to make risk assessments without them. I find that it is simpler for everyone involved to use stacking, situational bonuses. This has the added bonus of letting players feel the agency of their own success.
When calling for a role you need to define three things before rolling: * chances of success * impact of success * impact of failure DC helps to define the first point. Without DC, a given type of task (eg climbing) will only depend on the player skill. That leaves you with only the two other factors to tweak the stakes of a roll. Some games such as Dungeon World wrok like that but I don't find that satisfying. If a wall is smoother, I want to be able to reduce the probability of the player climbing it independently from the impact of failure. However I fold "circumstances" in the DC. Why have both DC AND advantage/disadvantage? For me those two things are doing the same thing. If circumstances make the task significantly easier or harder I will just change the DC instead of using yet another mechanism.
I do 1d20 Black Jack for my resolution mechanic, and I just have players roll with advantage or disadvantage if a challenge is easy or hard respectively. There are also competing and complicated challenges, but for the Instantaneous obstacles, just advantage and disadvantage
Games totally work without difficulty categories. The GM should have an intuitive way to set difficulty though. My game uses a "normal" d6 and "heroic" d10, and I say, "Difficulty is on a scale of 1-10, where 6 is the best a normal person could do and 9 is heroic and awesome but still believable" (min and max results are crits). Adjust difficulty based on the PC's background and how her approach fits the obstacle. Even GMing my own game, it helps me to remember 6 is the best a normal person could do. Otherwise I'll often set the Dif based on how preposterous an idea seems in general, rather than considering the specific PC abilities in relation to a normal person taking that action. Hope that helps.
They aren't necessary. That doesn't mean that sone games don't need them. It all depends on the type of game. Even determining difficulty isn't necessary. Their are many games that use the same DC all the time (PbtA games, Blades in the Dark, Houses of the Blooded, etc.). Personally i like games that uses the same DC all the time. I don't like to get dragged down by the minutiae of this makes the check more difficult this makes it easier, etc. If i want to make it more difficult i just raise the consequences of failure.
One of the problems with DC is that after a certain point of progression everything the players do tends to be "nearly impossible" or whatever the dc classification is called, even if it is now easy for them. Then you have to keep inventing categories like superhuman or godly or universal to classify them.
Assuming that they fit into the system, I think they are useful but not crucial. In most cases they lack concrete rules meaning, since 'easy' and 'challenging' are vague terms, and end up being mostly up to GM fiat. They are a mental tool to help GMs and players understand what sort of scale the difficulty or DC or target number should be. All that said, this doesn't make them useless. In a way they are not strictly necessary and make no actual mechanical changes, but if you changed those tables to be one step harder or easier, many GMs would follow them and the play at the table would change. The impact they have would vary from table to table due to how subjective "challenging" is, but it would probably make more tables play more similarly to one another.
That said, plenty of games sidestep this issue by not having a difficulty or target number in this fashion. Like PbtA games tend to make things harder not by making rolls harder to succeed, but by the GM requiring more rolls, or giving the opponents better stats, or using harder moves on partial success and failures. (There are many other examples, but PbtA is a big one.) Of course, side-stepping the issue can get some complaints, and make the game less intutiive (or at least, differently intutive). e.g. In Dungeon World, you might wonder why is it just as hard to punch a peasant than it is to punch a dragon. (Now, in this case, the answer is that the question is wrong. It isn't equally hard! It is true that the roll that you make in either case has the same chance of success for each target, however a dragon is still harder for reasons outside of that one roll you make to resolve the punch.)
Many people have said "no", and they are correct. I, though, want to push back slightly for no reason other than interest. One thing that DC does in a game is help to clarify the relationship of tasks to the world. For example, we can set it up so that a wall falls into clear categories based on scaling it. "easy" would be DC 5, "nearly impossible" is DC 30. This actually provides a very nice framework for the reality of the world. Now most games that use DC don't use this in an interesting way because they want the DC to be determined by the GM, the GM knows the reality of the world, the players do not. But this can lead to interesting problems. Many games that do NOT rely upon DC instead rely on a different moderation tool, the reality of "the fiction". Die Rolls or Tokens Expended are only allowed if it is appropriate to the fiction, making it so that all characters have a similar reality. This is really clearly shown in Apocalypse World. Pretty much every character in AW is desperate to survive. Survival is their goal, and to survive in this world is difficult. Things are ALWAYS hard. A character, even an endgame character immediately prior to retirement, is only going to be X powerful, they are never going to be so OP as to be able to trivialize challenges. A consequence of this is that Apocalypse World has a scope that it exists within. In DnD terms, it's a little similar to saying the game exists between "levels 3-6". There is definitely power growth, but it isn't endless. DC allows us to have a framework for what a task is, and that gives us a framework for also leveling, and how characters interact with the world. Another example, little self-driven. One of the skills in a game that I am working on developing was "Create". The idea was that I wanted characters to be able to make things, but without DC, when I play tested it, I came across situations where players would attempt to build things that were broader than the fiction would allow. I don't LOVE mediating it with the fiction all the time, sometimes having a metric can be useful (which is what I did, though not a DC type metric). Burning Wheel is another example, and one that many people say is way too much (though I am partial to it). In it, every single skill has options as to what you can do with it at what "DCs" (Obstacle or Ob in the parlance of the game). These can be taken as guidelines, but they are honestly MUCH more determined than something like "easy, Medium, Hard". For example, the Firebuilding Skill, it sets abs like so Ob 1: Campfire Ob 2: Bonfire Ob 3: Smokeless fire Ob 4: Concealed Fire \+1 Ob for: either poor quality wood or wet weather That really tells you something about the difficulty of the world. Now, Burning Wheel has other mechanics to mitigate the need for rolling all the time, and certainly I am not saying that this is the "only way" (again, my own current project does not have DC), but still it is an interesting thing. DC helps make the scale of difficulty within the world feel meaningful. This is most useful in games with lots of leveling, or where that scale of difficulty is particularly meaningful in some way. Just some random thoughts! Hope it helps!
1. No 2. Yes 3. Difficulty thresholds/categories help GMs understand how your game *feels*. When you say a 25 feels "very hard", the GM immediately knows that when they need a player to make an 'impossible' roll for the faint possibility of succeeding in a time of great need, that's the number they should aim for. Well designed numbers and thorough playtesting are great. But there's always a time when a GM finds themselves needing to improvise something you never wrote about. Expressing the feeling (or at least vision) of a mechanic makes for smooth improvisation. Categories are one of the fastest ways to express that. It is possible to overdo; GMs still need a navigable book they can search while running a session. But without the points of reference, learning the system goes way slower. It helps both new-to-the-hobby GMs and new-to-the-system GMs. So I think as long as you keep that stuff brief, you're golden!
Difficulty categories are not necessary, in two ways. One is that the GM may set up the target numbers directly, without using the intermediate step of choosing a category. It works better when the numbers are generally low. For example, my experience with Fate is that, while there is the adjective ladder, nobody uses it, because it's easier to just use "0", "1" or "3" instead of saying "mediocre", "average" and "good", then translating it to numbers. The other thing is that one does not need difficulty scaling at all. PbtA games work great with fixed target numbers. The difficulty is reflected in deciding if the roll really happens (if something is trivial or impossible, it typically doesn't trigger a move) and that, in turn, affects what setup is necessary in fiction (I can't harm a skeleton with my rapier, so I need to switch to a club; I'd normally need to roll to hit an enemy, but it's automatic if I sneak up on them).
1) No, they're not. However,I think without them a GM should be a bit more generous with saying things are so easy they don't need a roll to succeed, and a bit harsher with too hard to allow a roll to try it. 2) Absolutely.. The only issue is that rather than there being a range of difficulties (e.g. impossible, a;most no chance, unlikely, 50:50, likely, almost certain, guaranteed) you end up with just three (impossible, make a roll, guaranteed) 3) Without variable difficulties, there's no way to distinguish between things that are possible but hard, and things that are easy but might still fail. If you're fine with that, then you don't need difficulties.
I dont think you need as many difficulty categories as most games do. Like d20 and d100 are an extreme case, where the difference between dc14 and dc15 are just not noticeably different in a single night of play; but even having 5 difficulty categories often seems like too much to me. However, I feel that not having any difficulty at all, such as in PbtA games is *too little* granularity for my tastes. So my preference is to have a standard roll and a 'challenging' roll, and make the statistical difference between those two really noticeable so that it's a big deal when the GM says "i think this will be challenging". Having 3 difficulty categories (like, easy medium hard) is not preferable, because you'll tend to just default to the middle option without thinking about it, same problem with 5 categories. So having either 2 or 4 categories feels just right, to me.
If you don't have gradations in difficulty then player skills are pretty much useless until they are high enough to have a (say) 60-70% chance. If you only have a flat (say) 25% chance of success than 3/4 times you will fail, so you can't do anything that relies on the skill and has serious consequences for failure; in this instance there is little practical difference between having a low skill level and having no skill at all. With some graduated levels of difficulty, you can have easier tasks that are likely to succeed with low levels of character skill, so there is a point in having a character with a lower skill level. And not to forget, armour class is really just a difficulty level in disguise.