This is something my group has discussed from time to time, and it came up in another forum so I thought I'd broach the subject here.
I have most often heard the term munchkin used to describe a player who is always trying to min/max their character and is primarily interested in getting as many powerful magical items as possible. Note the difference between the munchkin and the Monty Haul campaign, where a DM gives out too much power too quickly or easily.
The negative concept of a munchkin, I believe, is an unrealistic expectation imposed by who knows when and who knows where. I think it exists to create a false divide between people who consider themselves to be "better" roleplayers than others (and a superiority complex in D&D is a pretty weak thing to have).
I posit that munchkins are, in fact, often better, more realistic roleplayers than those who deride them. Consider this: if you really were your character, and you made your living performing regularly life-threatening tasks, would you not want to have the best possible equipment for the job? If it meant the difference between living and dying, would you not opt for the least fair method of doing battle?
Imagine a soldier given a choice between a .38 revolver and an AK-47. The soldier will choose the rifle in probably all cases. Then, imagine that the choice is between the AK-47 and the same weapon, but one that would magically never jam or run out of ammunition. Clearly, any military in the world would leap at the chance to have such a radically superior firearm, and soldiers would do everything they could to acquire one.
Munchkins, players who try to min/max every last detail, are behaving more like their characters would in a real life situation. Even players who focus on non-combat aspects will max out their diplomacy and related skills for the same reason-- in real life, intelligent people do everything they can to be as effective as possible at whatever their task is.
Furthermore, players who don't want to play characters with average stats are probably being more realistic as well. Consider how many people in the world do truly exceptional things-- you can even include works of fiction as well. Can anyone honestly think of any examples of a typical, average sort of person consistently placing themselves in harm's way as a way of making a living? I'd be more than willing to bet that the average firefighter has some much better stat scores than the average person. I'd be equally willing to bet that a Navy SEAL has MUCH better scores than a +5 bonus overall-- and those are the types of people who become combat experts, not random accountants off the streets. Similarly, even for non-combat things, it takes MUCH more charisma and intelligence to be a masterful politician, musician, or even comedian than an average person possesses.
In any martial art, people with average abilities tend to end up as mediocre fighters-- just as people with average abilities tend toward mediocrity in ALL aspects of life. Having a character who has slightly better than average scores means that the highest they can hope to achieve is being slightly above mediocrity.
So it seems to me that players who accept that a real person will do everything they can to maximize their chances of survival, and who also accept that average or slightly above average people rarely accomplish more than slightly above average things, are the ones who truly are playing their characters correctly in their role.