Few things irritate me more than spending hours and hours preparing a plot plot outline in a particular setting in which what's available has already been defined beforehand, only to set it before the players and have That One Guy throw a tantrum because he can't make his Tainted Half-Dragonborn/Half-Shifter FactotumNinjaSorcerer concept work within the parameters of the campaign.
And it's always That One Guy who does it. Every group has one--the guy who gets his jollies from playing nothing but oddball character concepts and rebelling against anything he defines as "too Normal." It's like he's going through some sort of personal identity crisis and has this driving psychological need to set himself apart and constantly remind everyone around him that he's not just another sheep in the flock like they are. He demands the spotlight, often in social situations within the story, and gets off on all the drama that surrounds NPCs being revolted by his unusual and generally monstrous PC, detracting from the fun of everyone else around the table, who probably just want to get on with the campaign's storyline and make some progress towards achieving their goal. He just wants the whole campaign to be about everybody celebrating his diversity.
Tell him "No," or "You can't do that," or "That's not available," and he flies off the handle, wanting to do it all the more now because he was told that he can't, adding the appeal of rebellion to the appeal of the Weird.
I don't get this guy at all. To him, D&D is something very different than it is to me. He hates anything and everything old and traditional; if it's not cutting-edge and totally whacked-out Strange, it can't hold his interest. What made him this way? What was his early experience with the game like? What turned him off so much on the Things That Make D&D What It Is?