Cofounders need:
1) Complementary skills. This means a broad enough combined skill set to understand the market, build something and gain initial traction.
2) The focus/discipline to actually get things done. Most teams flunk this test completely, or one founder ends up trying to do everything. Either way, doesn't last.
3) High-level agreement on vision. Fighting about how to get from Point A to Point C can be useful; fighting about what Point C is? Not so much.