My two hard-and-fast rules:
Never date the insane
Never date the stupid
The first one came from an experience in college, and was reinforced with some other's experiences which I was relayed secondhand.
The second one... as much as you might think it might have entertainment value, it wears off really quickly when you're stuck trying to make conversation.
Some less-than-hard-and-fast guidelines kind of center around similarity of experience. I prefer dating someone who has similar economic level, intelligence, sense of humor, sociological upbringing, and other similar experiences. Differences are sure to be there in any case, but I'd have a really difficult time finding something to talk about with someone who grew up (for instance) as an Eskimo or in a South American rainforest tribe.
I'd also have a difficult time seriously dating someone who had strong ideological differences than mine if they weren't open-minded enough to tolerate a differing opinion. I'm thinking political or religious issues mainly, though there are other things that people can be passionate about.
I've also discovered that I have a hard time retaining interest in a woman that doesn't have a creative bent that she's passionate about. I really like passion. Passion and talent. Because passion without talent is very, very sad.
And smoking. Dear god, no smoking.
So what are some of your things? What are some date-killers for you?