A couple days ago I finally got being a good startup founder down to two words: relentlessly resourceful.Flickr is a great example of this. The company started as a massively multiplayer online game, failed at that, and morphed, in what appears to be pure grit and determination to succeed, into a photo sharing service.
Not merely relentless. That's not enough to make things go your way except in a few mostly uninteresting domains.
In any interesting domain, the difficulties will be novel. Which means you can't simply plow through them, because you don't know initially how hard they are; you don't know whether you're about to plow through a block of foam or granite. So you have to be resourceful. You have to have keep trying new things.
Be relentlessly resourceful.
More than any other characteristic, I think this is what to look for in the founding team of a startup, determination, flexibility, and willingness to do what it takes to succeed. As Paul says, be relentlessly resourceful.
Please see also Paul Graham's recent post, "Startups in 13 Sentences", that offers some other excellent advice for startups.
For a darker view on being relentlessly resourceful, please see my Nov 2006 post, "Ruthless enough for a startup?"