It drives me nuts how much some people suck at reading comprehension, because it forces you to have to put stupid qualifiers everywhere to account for their endless ability to miss the point. You have to stick a line like "I don't mind [x] in general, but...", or "it's just my opinion, but...", or "some", just to account for people who would start stupid tangential arguments (the kind that begin with "oh, so you hate [x] huh?")
The other thing is people who come onto a thread, just to tell everyone how much they don't care about the thread's subject. If you're not fucking interested in seeing another feminist thread, why are you here telling us that? Do you really think we wanted to know how disinterested you were with our discussion? Spare us and say nothing.
EDIT: Oh, I thought of another one. People who still dismiss people with the word "sheep", or any other term that implies something to its effect ("open your eyes", "appeaser", "believer" etc.) Yes, it still happens.