The idea should come first, the startup should come second.
For most software startups, this translates to keep growing. For hardware startups, it translates to don't let your ship date slip.
Cofounder relationships are among the most important in the entire company.
Great execution towards a terrible idea will get you nowhere.
In addition to relentlessly resourceful, you want a tough and a calm cofounder.
Most startups are not nearly focussed enough. They work hard...maybe, but they don't work hard on the right things.