Ridiculously loud kissing for an inordinately long amount of time. There have been vids I've seen where that kind of kissing goes on for over 5 mins. A little is OK, but when it goes on forever and is so loud, it's just a real boner killer.
Also like others have said, any sort of degradation. Just no.
And also pissing, spitting, scat, most food related porn too.