Oh man do I ever have a laundry list of this stuff.
Single-minded geeks: Just because I happen to enjoy a good game of DND every so often does not mean that I want to talk to you about it every time we run into eachother. There are other things in life.
Magic the Gathering: Admittedly it's more the people who I've known who play it. Namely a specific group of people at my old high school, all of whom had incredibly elitist attitudes and pretty much never showered.
Anime Fangirls: Yes I like anime, but I find it very difficult to get along with most anime fans, especially the girls. To them I have a few proclamations to make: Sephiroth is a fictional character, stop creaming yourself over him. No I do not want to read your FF Yaoi Fanfics. And finally, learning a few common Japanese words does not make you any more Japanese than the rest of us.
Ultra-Competitive Gamers: Basically people who get so absorbed in any game, be it video or tabletop, that they remove all elements of fun out of it. As was previously mentioned in this thread, I don't particularly care about every minor detail, I want to have FUN playing a GAME. Thank God I've got a sane group of people to play DND with.
LARP: God I don't even wanna touch this one with a 10-foot pole
Elitist nostalgia: Yes, the SNES and a good majority of its RPGs were awesome. However, automatically discrediting anything to come out since is completely stupid.