At first, when you don't know the person, you don't have anything but his appearance to define if you're interested or not. Online, you can try to also base on the things he wrote on his profile, but mostly, it's his physical attributes that will influence your judgment. Pretending otherwise would be completely hypocrite.
In my case, a charming face is the true basic, soft hands, abs and veiny arms are a huge plus. But in the end, what really matters, is that he has a sence of humor (a little bit crazy even), kindness, free and open mind. Someone I can share silly inside jokes AND intellectual conversations. I want a good sexual chemistry obviously, but in my case, it's intertwinded with emotional connection.