Well now this is an... Interesting thread. Why not, here's my list:
(1): Cheating on me is clearly the biggest offender. I'm generally a pretty forgiving guy to those I love, but if you decide to sleep with someone else, you're out on your ass. I'm sorry, but in an intimate relationship, romantically and sexually speaking, I had damn well better be your one-and-only for the duration of the relationship itself. Because trust me, I'd hold myself to the same standards.
(2): Being racist, sexist, or generally discriminating is another sore issue with me. I can tolerate making fun of my religion to a pretty large extent, since hell, even though I happen to be religious, I'll go that route sometimes, so if I feel its fair game for me to go that way, you can too. But when it comes to stuff like racism? Fuck no.
(3): Weak and overly-timid personalities are not only a turn-off, but also hard for me to mesh well with, in any kind of relationship. Give me a girl that's loud, proud, and headstrong. Of course, being physically strong would be nice too, but by no means required. But, a girl with a strong personality is not only nice, but pretty sexy.
(4): They need to communicate with me. This is not only due to the fact that, hell, sometimes I just want to talk to someone, but a relationship cannot work without communication. Did I do something wrong? Tell me, and tell me why. If I don't know, I can't prevent myself from doing it again, or defend myself if I feel justified in doing what I did. If they like something? Tell me! I want to know these things, so that way a relationship runs more smoothly. It wont be sunshine and rainbows all the time, god no, but it can be that way most of the time if we communicate and things work out.
(5): They need to be fair to themselves. I've met people that have been too harsh on themselves in general, as well as those that think they do no wrong. I can't really stand both, so I'd like someone who is fair on themselves when they make a mistake, or if they're an artist of any kind, when they judge their work or skills.
Now, ideally, they'd be a gamer too, and hold at least somewhat-similar interests. And other than what I've listed, I can't think of anything else just offhand.