If it takes more than a sentence to explain what you are doing, it's almost always a sign that what you are doing is too complicated.

A board member of mine used to say sales fix everything in a startup, and that is really true.

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

Mediocre founders spend a lot of time talking about grand plans, but they never quite make a decision.

You need to have a culture where people have very high quality standards in everything the company does, but still move quickly.

You need conviction in your own beliefs, and the willingness to ignore others naysaying.

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