Neat freaks, hypocrites, people who have a "cause," Most people who have a type A personality. Hipsters. Those people who lord everything over everyone else. Anyone who turns everything into a competition. Close minded or dishonest people. New Agey people too.
I try not to let people stress me out though, I worry about it because if I think too hard about it I make huge lists of people who stress me out (see above, I had to stop myself) I'd rather just get along with anyone but I get stressed when I can't, even if they're a wanker.