I would say a similar intelligence (which is psychologically proven,) similar beliefs and similar interests. Being bisexual, that works well for me but I am curious what gay men/lesbians on here think about this topic.
Similar intelligence: It is a way to hold good conversations together. Intelligence also means emotional intelligence, which is the ability to cope with stressful situations and calm yourself. If you're lacking this, conversations will likely become lopsided with one partner feeling left out.
Similar beliefs: Political, religious, philosophical, doesn't matter. If you have little synchronicity then it won't work. Some say opposites attract, but it takes obstensive argument and a large amount of compromise, which isn't healthy.
Similar interests: Well this is an obvious thing for us aspergians. Most wouldn't consider this though and go for looks.
I can't love someone physically before finding their personality attractive. You guys/girls?