People who don't want the responsibility of making a decision. Which is kind of funny, since I'm dating...well I'm not sure if we're dating, actually, but I'm going out with a guy who's kind of non-committal about everything. And people who are too nice, and pretty much let you walk over them.
It's great if you're a kind and respectful person, but it's not so great when you have absolutely no opinions. On the other hand, I also dislike guys who walk around expecting women to fall in love with them. And guys who can't take a joke.