Much discussion of why people despise a given edition of D&D is given over to issues of game balance-- as in "in 3.5 edition, clerics and druids are unbalanced" or "4th edition is terrible because everyone is balanced against each other". IMO, the issue of game balance is important, but not in the way most people think.
Playing an RPG sucks if you spend an hour sitting around with nothing to do, or with nothing to do that wouldn't have gone the same way for everyone if you weren't there. Then, it becomes a wasted hour AND you feel like you're being screwed for running the character you thought would be fun to play. It means no fun for you, and a mild persecution complex.
So, I propose the issue is less a matter of balancing power than it is a matter of balancing interesting stuff for you to do. Think of it as balance of screen time. If a player can't affect combat BUT has an essential role in the rest of the game AND the game isn't all about combat (with two hour combats), then having (for instance) a wizard who can only cast one spell isn't so bad -- as long as you know you can also be the one who gets to solve problems and affect roleplaying situations and generally show off at other times. Being a wuss in combat is okay IF you kick ass elsewhere AND have opportunities to shine. But if you might as well have spent the day at home doing something more interesting rather than watching your friends have fun gaming, then something is wrong.
I would also propose that as D&D combats have become longer (as they have in 3.x and 4E ) this sort of spotlight balance is harder to achieve, since combat may be most or all of the action that occurs in a given game session. In the case that screen time becomes close to or actually synonymous with combat, balance of screen time starts becoming the same thing as balance in combat, which may be where the sense of dissatisfaction will come into play for those playing "weaker" characters.