EDIT: New one. When people on the Escapist constantly quote Yahtzee. You're not Yahtzee. You're not as funny as him. Shut up.
If I ever read something like "Oh, I never played it, but here is my opinion from what I heard about someone else" like a certain topic that's come up recently. If you haven't played it, you don't know what you're talking about. So you don't get an opinion that's valid, because you don't know. It seems simple to me.
I really hate it when my fellow nerds and geeks live up to the stereotype. Like I am in a Warhammer club (yeah, pretty damn nerdy, got an entire Battle Company and everything) and I really get embarrassed going there because loads of them do have some social problems, and body odour is a serious issue, and they really are overweight and dress very poorly or dirtily, and it would take so little effort to at least be clean and not stare at every girl's breasts when she walks into the room. Or when I walk into a video game store with my girlfriend and everyone just mouthbreathes at her and I actually feel shame to identify with these kinds of people. It's the same reason I am no longer a brony; after a point, it's just kinda cringeworthy.
I very much dislike the whole "The older games are always better" argument. I personally believe that Final Fantasy 9, 10 and XIII-2 are better games than 7 or 8 in terms of characterisation, and that Final Fantasy 12 is a perfect example of why I don't like the first few Final Fantasy's, because they are unfocused sloppy messes with no particular character I like. And I think Final Fantasy is heavily overhyped, because while it is VERY GOOD, it suffers from having a very unlikeable protagonist and a broken combat system which removes challenge after a while unless you chose to fight the 3 Weapons.
Spoiling a game which has a major plot point for being spoiled. E.g. Bioshock, Walking Dead, Spec Ops. If you can avoid it, then don't mention it and ruin it for someone.
I dislike fan service and harem plots in anime. Because it leads to a sloppy uninteresting anime where no progress is made with anyone so no fan gets upset. And fan service and slap stick comedy is just no.
Farting. It will just turn me straight off anything. I'm not asking for biting satire with every joke, but if your humour literally comes from your arse, then your material is close enough to shit to be a turn off.
Changing Snake's voice from David Hayter. That's petty, but Snake should be gravelly, dammit!