I find urine, speculum play, extreme deepthroating, and bukkake to be gross and perverse. I don't like unnatural types of porn where people bust out all these weird medical-type instruments, or concentrate on humiliating a performer. There are definite perceptible atmospheres (mood, ambience) to scenes where you can tell there's not much passion or "heat," it's merely about being as weird or cruel or perverse as possible. That's why I like performers like Mike Stefano and Manuel Ferrara, who are passionate about women (although they're both plenty dirty!). It's nasty and wild and hot, but it's most often NATURALLY nasty, wild, and hot.
Piss drinking and shit eating gross me out (believe it or not there is actually a place in the Netherlands that puts out shit eating videos). I also find blood, needle and piercing play to be nauseating (don't know of any porn companies putting this play out but it can be seen in BDSM dungeons, Stefanos and Shay have classes in SF on it). Believe it or not there is actually a sect of BDSM where people are pierced by large hooks and suspended from the ceiling by them!