The barebones:
1. Common vision : Do you share the same vision, or are you at least able to bridge that delta in the individual visions?
2. Value proposition : What do they bring to the table? Capital? Hard Skills? People Skills? Business Connections?
3. Mutual Trust : When the situation calls, is either of you comfortable with letting the other chap run entity entirely on their own for a while?
A while back, Idd Salim blogged some thoughts in relation to this, albeit from the Techie's standpoint:
http://bit.ly/12y8GQT
Tony.