There's always Talislanta, a sorta high-magic fantasy setting, which, though it uses a D20, doesn't use the D20 mechanic. It's similar though.
On a d20 roll, a 0 or less is a mishap. 1 through 5 is a failure. 6 through 10 is a partial success. 11 through 19 is a full success. 20 or higher is a critical success. You roll and add modifiers, instead of having a set DC.
Tal makes a lot of use of the mishap mechanic because of the world's reliance on magic. Long ago, the excessive use of magic caused a Great Disaster that damaged the whole world, leaving the world to recover (which it has), and weakening magic. Though magic is very commonplace, it has restrictions. A given spell can only do one type of thing (attack, or defend, or illusion, but not a combination), and any given person can have no more than 7 magical enchantments on him at once, or else none work. Plus, though you can conceivably try to cast anything, all spells have a chance of mishapping, so you don't want to over-exert yourself.
Great magic system, and it's simple and adventuresome for everything else too.
