Most Helpful Guy
Depends on the girl or woman.
Sometimes it's something "physical (hair, eyes)," sometimes it's "visual" (How she carries herself, her sense of style, etc), sometimes it's a little detail or mannerism (though usually this is coupled with something else), and sometimes it's hearing that we have a lot of things that we have in common. It's usually a combination from "all of the above."
The woman that I'm interested in now - she's very attractive - but not what I would have thought I'd ever go for. But it's not "just" that. We have similar interests; We've worked at similar jobs; we've had similar obscure pets; we have a lot of the same hobbies, similar tastes in movies and TV shows; even our "bucket lists" have similar things on them. The hobbies that we don't quite share 100%, I think that we appreciate them, but for different aspects. It's very cool.