I feel like above all, the person should fit in with your group. By that I mean, they should have good character, similar morals and worldview, similar style in music, and easy to get alone with. Their personalty should also fit your band. (For example, if you have a headstrong choric in your band, it's probably not a great idea to get another headstrong choric.) Obviously they should be skilled musically and in their instrument. But it is a lot easier to improve musically or stylistically as opposed to changing someone's personality, which you likely can't.

The reason this is so important is because even if you have an AMAZING person musically, but you guys clash as people, there will always be tension, strife and conflict in the band; thus making it hard to go anywhere. However, if you have a decent musician that you get along with and enjoy as a person, that individual can always improve themselves musically. And on that note, they should always be teachable, that goes back to character. You don't want someone supper arrogant or stuck in their ways.

Bottom line, talent is good, but character and personalty is key.

Hope that helps!