We talk to a team they've gotten new things done, that's the best predictor we have that a company will be successful.
Be suspicious of any work that is not building product or getting customers.
In YC's case, the number one cause of early death for startups is cofounder blowups.
Growth and momentum are what a startup lives on and you always have to focus on maintaining these.
In general don't start a startup you're not willing to work on for ten years.
As you grow, the productivity I think, goes down with the square of the number of employees if you don't make an effort.