People who complain, mock, or ridicule RPG systems that they have no experience with. I've been playing Palladium RPGs and the Hero system since I was in middle school and I love both of them. I get that they may not be the most popular gaming systems out there(especially Palladium) but I enjoy them both very much.
What bugs me are people who say things like, "I've never played Champions but I know it's super complicated and requires an advanced mathematics degree to play and that's just stupid." Or some such. All they do is parrot garbage they read off of forums and act like this makes them experts. Then, when you try to discuss this with them, just just shake their heads and walk away. It's very irritating.
I'm tempted to make a Champions or Heroes Unlimited MUSH just to spite people.