In YC's case, the number one cause of early death for startups is cofounder blowups.

I care much more about the growth rate of the market than it's current size and I also care if there's any reason it's going to top out.

The tenth social network, and limited only to college students with no money, also terrible. Myspace had won.

There are exceptions of course, but most companies start with a great idea - not a pivot.

... fire fast when it's not working. It's better for the company, it's also better for the employee.

A lot of people treat choosing their cofounder with even less importance than they put on hiring. Don't do this.

By the way, that's my number one piece of advice if you're going to join a startup: pick a rocket ship.