Ahh a topic very close to my heart. Attitude is the number 1 attribute to look for. The developer needs be able to problem solve, be prepared to work long hours, is either single or has a partner that understands what it takes to succeed in a start up. Likes to be part of a team and either has business nouse or at least understands there are business imperatives not just coding ones. Ideally their ambition is to code or at most manage good coders and to be part of team that runs a successful business. Finally, good solid coding experience that is well designed, commented and documented. Oh one more thing. You need to get on with them as a start up partnership is fun, tumultuous and rewarding as a marriage.