Common interests are vital, you've got to be able to talk to the other person without boring them. For me, compatable interests include- Video gaming, rock/metal music, anime/manga, cinema, comedy, literature, and internet culture. It would also be ideal if they had some acedemic interests of sort, thus being intelligent enough to discuss academic topics with. For me, the humanities most interest me. I'm alright with sciences, but humanities are preferable. Provided they have some academic interest, then i'm ok. If they have a creative interest, like playing instuments or writing stories or poetry thats great.
Ultimatly, you just want someone who is interesting, and thus you will be confortable spending time with them, and essentially get on like a house on fire.
Cognate personality- their personality has to match with my own. I could not really go out with one big highly social extrovert, since i would feel forever in their shadow. Ideally, they should be reflective, thoughtful and to an extent introverted. That way your on an equal level with them, and you can easily relate to eachother.
Other features- practical things like where they live are important, you obviously need to be able to get to them easily enough. Physical attractiveness as well, since thats the first thing you notice about them in the first place. It also helps if you get on with the rest of their family.