1. Chubby/Overweight - I have NOTHING AGAINST chubby guys, but when it comes to pr0n, I prefer leaner meat.
2. Obviously staged - I don't like pr0n where you can tell that it's all plastic garbage; goes with what raphjd said about the people saying things like "Take it bitch" or "Fuck me harder"… It all just seems unnecessary and silly.
3. People that look obviously under 18 - I know some people love it when someone is 18 but looks 16, but honestly, it's not for me. Anything even remotely resembling pedophilia is a huge turnoff for me. Kind of ironic, because I've been into older guys since I was 14, but there you have it.
4. Half-flaccid cocks - I still don't understand why people in pr0n always seem to not be totally hard. Maybe I just get abnormally hard when I have a boner and that's what I'm used to, but floppy dicks don't do it for me. Long hard stiffies, though, definitely do.
5. Rimming/Close-up shots of assholes - I'm just not into rimming; the close-up shots of assholes isn't so bad if there's a cock stuffed in there, but otherwise it's not my thing. (I've never actually tried rimming though, so that may change if I ever get any experience. :P)
I'm sure I have more, I'm rather picky about my pr0n, but I've stuffed up the thread enough. XD