It's become popular in recent years to say that the idea doesn't matter.
You need unstoppable people. You want people that are just going to get it done.
You have to be decisive. Indecisiveness is a startup killer.
More important than starting any startup, is getting to know a lot of potential co-founders.
Because so few people make an actual long term commitment to what they're building, the ones that do have a huge advantage.
Every thing at a startup gets modeled after the founders. Whatever the founders do becomes the culture.