I love having a variety of classes in a game. One fairly easy way to allow a lot of variation from character to character is to allow multiclassing. With multiclassing, there don't have to be as many base classes to provide the same amount of options as a large number of base classes paired with no multiclassing. A game just really needs incentives to be creative and try different combinations of classes. These incentives could be as simple as some skills that can only be acquired through certain class combinations.

Dungeons and Dragons is a good example of a game with multiclassing, but I don't really think there is all that much incentive to multiclass besides the prestige classes that you can attain at higher levels.

Other than multiclassing, I'd like to be able to make a finesse fighter. My current Neverwinter Nights 2 character is a fighter/rogue, and I love the combination. I've also started leveling as a duelist which is basically a prestige class for finesse fighters.