The biggest causes I've seen of early startup failures are founders giving up as soon as the going gets tough, and fights between co-founders. Therefore, I think determination is the most important quality in founders and there must be a trusting relationship between them.

Founders also need to be basically level-headed. They need to think big enough to have crazy ideas but not to be themselves deluded about how hard it will be to pull them off.

I'm collecting interviews now for another edition of Founders at Work!