I'm changing the rules for conflict resolution in Siege from a roll-under to a roll-over idea. The maths changes without difficulty so all probabilities remain the same. The big achievement is that roll-over rules are more easily understood to players and this takes the rules out of the way of the fiction.
I like to think that rules are necessary, but they should never overshadow the fiction. Bring the rule in where it's required, but make sure that sends the players back into the fiction quickly. A less-convoluted rule helps to make this happen.