% Systems? (you have an X% chance of success, pass/fail) -- Call of Cthulhu, Palladium System (for Skills)
Additive Systems? (Roll Stat + Skill Dice, total the value. Roll X dice and add Stat + Skill, total value.) -- AEG's games, WotC's games, 1E to 3E Shadowrun
Success Based Systems? (Roll Stat + Skill Dice, count anything over X.) -- White Wolf games, 4E Shadowrun
I'm a fan of the success-based system myself. I've often found % based systems to be unrealistic, and additive systems tend to get broken at later levels (minimum value soon surpasses the base DC for a'hard task', thus disallowing failure). Success-Based systems seem to operate better, allowing chance of failure even with a large dice pool, and scaling how well the character succeeds. These two things, I feel, set up the best dice mechanic I can think of.