Eh, it depends. Generally though I'm more likely to approach people who I don't think will tear my head off for talking to them, lol. I.e. people who look open and friendly. Stoic and mysterious is one thing, but looking super sullen and angry tends to put me off unless you approach ME and show that you are not indeed, sullen and angry.
I don't know any guy liek this who actually is good with girls.
For real... its best to look relaxed and approachable. Be nice but not the 'nice guy'. Don't talk too much abou yourself and listen to girls, since I act that way, I do not only get really nice chicks at parties, I'm way more popular at school, got to know nice people and stuff.