Uniform probability versus binomial probability. I'm better at math than the vast majority of gamers I've known and I can't get an idea of the probability of rolling a given number of success. Actually, scratch that. It's not a binomial distribution. A binomial distribution would require only successes or only failures. I aced Prob and Stats in my sleep and I have no idea how to calculate those probabilities; how the hell is a teenager supposed to figure it out? Yeah you can figure out the expected value of any given roll easy enough, but what if you need 3 more successes than that? In the d20 system, only a few weapons and abilities use binomial distributions. Almost everything is uniform, which means on the fly you can say "OK so I need t roll a 16 or better so it's a 1 in 4 shot." Instant comprehension of what the character can reasonably expect to achieve.