There's been some discussion among my RL friends, of late, about the sorts of boundaries people are unwilling to cross, even in roleplay. I, for instance, will not PC--NPCing is different, naturally, as is playing as a villain in a one-shot LARP--a character I can't respect. This doesn't mean that I always play squeaky-clean hero types, but it does mean that my characters at least adhere to my quasi-Viking moral code. All of my characters will try to stand on their own, will be relatively reluctant to make a public crying-type scene, and will try not to hurt innocent people to further their own ends. A friend of mine can't play characters without loyalty, while another can't crossplay.
So, what about you guys? What sort of characters do you find it really hard to play?