The premise
There are three things that count when trying to determine if a certain task is done:
+ The skill on such area
+ The difficulty of the task
+ The degree of success
While making a Dice/Cards/Whatever mechanic to settle the result of an attempt, what we do is assigning this 3 values as the numeric variables of the draw/roll/whatever.
For example:
d20:
+ Skill: a numeric bonus (higher = better).
+ Difficulty: a numeric threshold (higher = more difficult).
+ Degree of success: difference between 1d20+skill and difficulty.
WoD (pre Requiem):
+ Skill: number of dices rolled (more = better).
+ Difficulty: a numeric threshold between 1-10 (higher = more difficult).
+ Degree of success: Number of dices with a result >= difficulty.
GURPS:
+ Skill: a numeric threshold (higher = better).
+ Difficulty: a modifier to the roll (higher = more difficult).
+ Degree of success: difference between skill and 3d6.
A good system is one which is fast yet provides a good degree of realism.
Some notes
+ Dice counting is faster than dice addition. Even more when the added values are greater than 10 (when your fingers aren't enough).
+ Value substractions is slower than additions, and it's better to add elsewhere.
+ Multiplication/division are not encouraged at all. Dividing by two, rounded down, is a very common modifier, and slows down everything.
+ One feels powerful when his modifiers are big, or when he rolls a lot of dices or scores a big number. Therefore, higher = better is good becomes it comes out intuitively.
+ Either you decide bigger is better or worse, keep that criteria along all checks on the system. Not doing it was probably the downfall of AD&D.
+ If the setting is associated with some way of determining random numbers, using it adds flavor to the game, such as playing Deadlands with a Poker deck.
+ Diceless decision: rolling to find out if you can tie your shoelaces is dumb. It's the same thing with a professional marksman who wants to hit a cow with a rifle 10 meters away. All the way around, it's impossible for the common guy to lift a car over his shoulders. The success-fail criteria of the system must contemplate this 100% win or lose situations. Are there imposible tasks, or just very very very very very very very very hard stuff?
Comments are truly welcome.
Keep on rolling,
Fernickk