From the data we have so far (which is starting to be a lot) the best predictor of success is determination. Then flexibility, intelligence, sense of design.
"... stubbornness is a disastrous quality in a startup. You have to be determined, but flexible, like a running back. A successful running back doesn't just put his head down and try to run through people. He improvises: if someone appears in front of him, he runs around them; if someone tries to grab him, he spins out of their grip; he'll even run in the wrong direction briefly if that will help. ..." ~ Hardest Lessons for Startups to Learn, 5. Commitment Is a Self-Fulfilling Prophecy ~ http://www.paulgraham.com/startuplessons.html
Number FIVE!
Just kidding. Maybe I read too much, but I find a lot of statements pg makes are right there in essays if you read them. I particularly remember this one because of the sport analogy, dodging, weaving around obstacles to get to the objective.
"... When you say flexibility, what exactly do you mean? ..."
Guess thats why it's on the list (hidden, but there) at number 5.
I'm curious...do you see mostly the flashy instantly-visible "I'm going to succeed at all costs" determination, or the kind of quiet resolve that only appears when things are going poorly and the founders choose to go on anyway?