The thing I hate most in a guy (And in people in general really) would be dishonesty, or a... reluctance to take blame (Which I qualify as an extension of dishonesty) I like to think I'm generally accepting of people's flaws and shortcomings, but I have absolutely no tolerance for pride or overconfidence, even when it's feigned by someone in an attempt to save face. For instance, when someone fails at a task or something, they then immediately attempt to justify that failure - "Oh, I'm not feeling well today" or "Man, that other person really screwed me over here!" Just own up to your failure, and try to move on. So I generally dislike people who try to blame their mistakes on something else, or build themselves up as better than they are. I mean, people screw up. We do it all the time. It's nothing to be ashamed about. And wouldn't it be better to undersell yourself, and then blow me away later with whatever awesome talent you've got? But on the other hand, I don't like dishonesty, even when it's well intended. If someone gives me a compliment or a kind word, I want it to be genuine, or I don't want it at all. I don't care for empty words and broken promises - which apparently sets me apart from society, since I've been called a jerk for refusing to give undue compliments.
Another thing I hate is intentional malice, even if it's in retaliation for being wronged. I can understand being upset about something, and I can understand wanting to vent that frustration, but setting out to purposefully wound someone is not something I can agree with. Hate doesn't get anyone anywhere.