[limist]

Character (integrity, drive to learn & improve) and skills that you respect or admire. Then shared values and life situation: the startup should be the #1 priority for everyone on the core (full-time) founding team. Missing any of those requirements increases the risk of failure.

Unfortunately for those without a wide/deep network of college friends, former colleagues, etc. it's hard to judge character and skills without a previous friendship or shared work environment, etc. If you're forced to evaluate a potential co-founder in the absence of shared history, work on something small together for a few days/weeks and see how it goes.

Another thing to do in all cases is to discuss downside/failure scenarios right up-front, and see if there's a sense of openness and fairness. Check out the books The Founder's Dilemmas and Slicing Pie for structuring the conversations, as they give vital ideas and precedents around formalizing the founders' relationships. It's much easier to chat about failure and equity before real work starts, and I've seen many times that it flushes out assumptions and behaviors that you won't want on your team (assuming you're the reasonable and fair one :-).