Interestingly, while reading this thread, it seems different people use the words in different ways. "Powergamer" seems to be derogatory to some people, but not an insult to others. Maybe some precise definitions for "optimizer", "min/maxer", "powergamer" and "munchkin" is in order here? Just a $0.02 observation, btw
IMHO, "bad character-building" (whatever term you use!):
- takes unfair advantage of unclear rules, loopholes, etc for power boosts; and/or
- completely disregards the spirit of the setting; and
- makes the game un-fun for others.
A powerful character isn't necessarily a bad character, as long as it fits into the game. This is why I like the give and take between the GM and player: creating a story of how a character evolves and gains that power - like the fey warlock mentioned above - makes the game vastly more entertaining for everyone involved, regarless of how powerful the character ultimately becomes (or doesn't become!).