iserith
Magic Wordsmith
My biggest pet peeve is that one player (you know who you are) that simply cannot, for the life of him/her, create a serious character. And by that I don't mean creating a character that has no personality/quirks; I'm not expecting every character out there to play out as a stereotypical Lawful Good paladin.
I mean that inevitably, whatever they choose to create, it ends up more caricature than character. Their creation could make a guest appearance in a Looney Tunes cartoon and no one would suspect the character hadn't been specifically created for that entertainment medium. They can often be spotted by their names, which are pun-filled, double entendres, or both.
Yep, that's me - the high concept character with the pun for a name. "Immovable Rod" Manleigh the brawler fighter with the professional wrestler persona. Chuck Dagger, my blade-throwing rogue. Wei of the Shadows, my shadow monk. Ebenezer Switch, the druid/transmuter alchemist.
The trick is to be effective and drive the game forward while being subtly comical. Disrupting the plot or being silly for silly's sake is when it gets to be a problem in my experience.