I think I gave this answer a while ago, but, I've never really had a problem with repeating myself.
The hyper-optimized, one dimensional character is a problem. It's no different than a Mary Sue in fiction - it' s the character that can do everything, and do it better than everyone else. It's a spotlight hog and in a game which does revolve quite a bit around the group, this is a problem.
OTOH, the one armed peasant with a spork is every bit as much of a problem. While a Rinsewind (if you're a Pratchett fan) is funny in fiction, in the game it's just as much of a spotlight hog as the John Carter. For one, it doesn't fit into the game particularly well because it doesn't fit into the group. Why is the group dragging around this dead weight? Generally the only reason is because the dead weight has the glowing PC halo floating above his head.
One of the biggest problems I see though, is when people who don't have a strong grasp on the mechanics start criticising other players for being "overpowered". I mean, that one shot character from several pages back that had to burn a daily, a couple of other powers and succeed on three attacks in order to do gobs of damage got called a powergamer.
Until you start looking at the math.
Sure, about 1 in 8 attempts (given a 50:50 chance of success) he's going to beat the living crap out of something. But, he can only try this once per adventuring day. So, on average, he's going to pull this off about once per eight adventuring days. Heck, the way we play, that's about once per level.
Step over to my minotaur fighter for a second. +2 vicious war pick (and no one is going to claim that THAT's optimised) does 8+d8+2d12+5 (str)=30 points on average on a crit. How many times per adventuring day does he crit? Even if it's only once, he's still doing over 200 points of damage to the "overpowered" character's 90 (on average).
Suddenly, the "overpowered" character isn't really all that big of a deal anymore.