It really turns me off when I feel like they're acting or not really into it. If they're too over the top and trying to act it's so distracting and if they're just not into it it's a real turn off. Also when you see a bottom that isn't hard.
Also these days I kind of find porn with condoms a bit of a turn off… Well.. not so much a turn off but if I see they're using condoms I'll usually find another video straight off.