Confidence. I've seen many ugly or not the best looking guys get gfs but they were confident. I have guy friends who are handsome fellas, yet they're too insecure and are afraid to approach women and date. Even if they get approached, which they do, they still feel too insecure to let go and date.
I think for guys though, it's opposite. They can "work" with a chick who isn't confident more than an ugly girl with confidence.
Not all men, and not all women of course. But I think it's more common.
Well, it depends. For very attractive men, confidence isn't as necessary. For very unattractive men, I think confidence isn't very helpful. For those in between (which is most of us), confidence is very helpful.