Personality. If we don't click in any way then I am going to reject you; if you're not good looking but we have a personality spark, you have a much, much better chance than a great looking guy who's nothing like me. Looks play into it, of course, because I'm not going to pursue you if I don't find you at least somewhat attractive, but personality can go a long way in making you attractive. More than once I've found myself attracted to a guy after talking to him, when the evening prior I probably wouldn't have considered going out with him.
As bad as this sounds, the first factor is looks :( even though men are more visual, women are visual too. You don't want to have to kiss all over someone who you don't think is remotely attractive. But personality is also a huge factor, nd can change a girl's perspective on a guy. A guy can be really sexy but not attractive because of his horrible personality. Likewise, he can not be as cute but be really attractive because of his personality.
Secondary factors would be things like morals, beliefs, favorite things, how you handle certain situations, etc..
Other factors are like personal style, hygiene, etc...