Growth solves (nearly) all problems

You can't be focussed without really great communication

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

You want to think about what is the path for my first 10 or 15 employees going to be as the company grows.

A related advantage of mission oriented ideas, is that you yourself will be dedicated to them.

You also want to fire people who a) create office politics, and b) who are persistently negative.

In YC experience, 2 or 3 co-founders seems to be about perfect.