I guess I'm going to repeat a lot of what other people have said, but here goes anyway.
With individual girls it's big/multiple tattoos, piercings (other than ears), fake tits or other obvious cosmetic surgery, spray-tans. I'm getting used to bellybutton piercings but I'd rather not see them; pierced nipples etc are still an instant turnoff for me.
With solo sets the worst are the ones that are just long-shots of the whole body over and over again. I also prefer the girl to start with clothes on in a "real" environment, not against a studio backdrop. Gimmicky outfits -- you know, santa suits, bunny ears etc -- switch me off, too.
With girl/girl it's lack of interest. So many "lesbian" scenes are incredibly stilted. If the girls don't seem to be into it I'm not likely to be either.
With girl/boy it's mainly the things people have already said: any suggestion of violence, degradation etc should be as off-limits as children or animals, period. This has spoiled a few GGG scenes for me; bukkake, if it's your thing, should be about worshipping the girl, not humiliating her or forcing her to do something nasty. As someone said above, the words "brutal" or "nasty" are a good sign I'm not going to like it. And by the way, I'm not talking about consensual BDSM here, which isn't my thing but I'm not knocking it.
The other things are more about personal taste. I don't find anal very interesting. I like clothes, teasng, a little foreplay: a scene that starts straight in misses an opportunity for me. There has to be a cumshot, and it annoys me if the film then immediately cuts -- I want to see the results. Too much focus on the guy doesn't work for me. Too much phoney "performance" is annoying and suggests the girl is bored and having to put it on. Scenes with girls who look too young, even if they're not, creep me out, especially when they're done up with pigtails and lollypops etc. No thanks.
I prefer not to have more than one guy in the movie. As someone said, while I'm watching that guy is me. I don't fantasize about scenarios where there's another guy. But I'm OK with you if you do
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