I only watch gay porn, so:
1. Tattoos. Hate 'em. Unfortunately, the type of guys I like -- masculine, 25+, white/hispanic -- almost all have tats, many of them all over. Wish there was a site dedicated to non-tatted guys, or at least they would add a tag for clips where the actors have no ink so I could find them easier. (Part of the reason Men at Play is one of my favorites is that the men are mostly clothed, so you don't normally see any tats.)
2. Spitting. I don't get this at all. Yuck.
3. Endless build up. Ok, I know some people like the slow burn stuff -- I have a close friend who loves that -- but me, I want to get right to the action; and by action, I mean fucking. So I'd love it if they added flags to the clips so you could get right to what you want without having to search the clip for it.
4. Femme/twink bottoms (or even tops). MASSIVE turnoff. I'm watching porn where MEN have sex with MEN, and I want to see a MAN getting pounded; I want nothing that smacks of femininity -- no mannerisms, no pierced ears, no nail polish. Conversely, he shouldn't be cartoonishly butch -- seeing ordinary-appearing but still-masc men get fucked is glorious.
5. Bottoms who jack off. Dude, you're there to get fucked, leave your cock alone. Your only pleasure is inside your ass.
6. Poor angles. There should be a good balance between close-ups of the fucking and wide shots showing both bodies; in the former, I want to see the cock going in and out of the ass, and if you're also showing the bottom's useless, soft penis, even better; in the latter, try to include the bottom's face, so we can see the joy he's experiencing.