Losing focus is another way that founders get off track.
Stay focused and don't try to do too many things at once. Care about execution quality.
A single mediocre hire in the first five will often in fact kill a startup.
Startups are very hard no matter what you do; you may as well go after a big opportunity.
It's difficult to get large groups of people, to the extreme levels of focus and productivity that you need, for a startup to be successful.
For early employees you want people that have somewhat of a risk-taking attitude.