Do they like a lot of different kinds of music and are they into music apart from just playing it?
This is something that's an absolute must for me. I don't really want to be in a band with someone that listens to nothing but power metal or nothing but punk rock from '76-'84, etc... I've found that people that are into and expose themselves to a lot of different music are more creative, open-minded, and easier to work with when it comes to writing and arranging.
+1000