The professionally insecure: "You wouldn't be interested in someone like me. People don't like me. I would like to see you again, but you probably don't want to". AAARRRGGGHHH! Always expecting you to tell them: "Of course not, you're fantastic!". I just ignore and move on. They need a therapist, not a date.
The horrible spellers. Spelling mistakes really put me off. needless to say, that used to rule out 3/4 of all chances when I was online dating.
Any reference to their cock in profiles, or in the first chat. Some men are absolutely obsessed with their cock. It's alright, but they need to understand their obsession does not apply to other people. I have seen many cocks, they're nothing special and yours isn't either.
The ones who expect that make up is a fundamental part of a woman. I don't use make up, never have, never will. If you don't like it, move on, but don't try to tell me what to smear my face with.
The ones who try to impress you with money. I earn my own, thank you very much, and it is often much more than what you can make. If I was attracted to money, I'd join sugardaddy.com