Originally Posted by Jay R
Consider two hypothetical characters. Xavier hits on a 2 or better, and Yancy hits on a 20. This is about the best and worst possible odds.
Regardless of skill, they each fumble 5% of the time, and they each crit 5% of the time.
Xavier Yancy only hits 5% of the time, but all his hits are critical. Meanwhile, all of Yancy's Xavier's misses are fumbles.
This is clearly ridiculous, and the probability of a fumble should go down as you get more experience.
I don't actually find it ridiculous, at all. It's very cinematic, which is the exact tone of D&D.
Xavier is the character who's awesome, totally awesome. He never fails to hit--except when things really, truly, catastrophically go wrong for him. Then he comes crashing down in a giant heap of hubris. Is it realistic? Of course not. But it sounds pretty much hilarious and/or dramatic to me, and pretty fun.
Yancy (btw, you flipped Xavier and Yancy in the second paragraph, fixed it) is utterly, totally incompetent. Yancy is the bumbling fool who can't ever seem to do anything right. And yet, every once in a blue moon, Yancy lucks out and pulls off something so ridiculously, awesomely spectacular that it makes me shed tears of manliness. That's the sort of moment where everyone goes "I can't believe you just did that..." and a light shines from the heavens, yadda yadda yadda.
So maybe it's ridiculous if you want realism, but it's totally awesome if you're looking to a traditionally cinematic and dramatic tone, which is what D&D was always structured around.
RE: balance, I totally don't think it's unbalanced. Tacking on a minor success rider for a crit sounds like a fantastic idea. My suggestion: dribble in a partial success from another, related skill. Using Diplomacy to convince the guard to let you through might also reveal a tasty rumor on a Crit that you would have had to roll Gather Information for. Or it's the sort of information that you'd have to do a lot of legwork and investigating to track down.
Really, I feel as if the players know "on a 20, your success goes from 'good' to 'utterly fantastic'", it's cool.