Ideal resolution systems for RPGs ought to have a large "sweet spot". What's a sweet spot?

Well, if you use 3d6 (the way GURPS and Hero System do) then your "sweet spot" is the range of "difficulties" represented by having to roll 8 or less to 13 or less... The probability curve is very narrow, which means that in GURPS the range of human capability ends up being pretty narrowly defined (a character with an ability score of 9 is pathetic, while 15 is awesome). In Hero System almost anything you want to do is trivial or impossible.

The ideal system would be exponential in generating probable outcomes, something along the following lines: if score A opposes score B and they are equal, A has a 50% chance of prevailing. For every point by which A exceeds B, the chance of failure is reduced by 5%. For every point by which B exceeds A, the chance of success is reduced by 5%.

Well that's RuneQuest opposed resolution, right? NO! RuneQuest success chance goes 50%, 55%, 60%, etc. I'm talking about 100 - 0.5 * 0.95^n where n is A-B. In other words, if A exceeds B by 100, going to 101 reduces the chance of failure by a further 5%.

Of course this is a pain in the neck for dice-based games, but it works jim dandy for computers :)