I think the founder's worst enemy is often his ego. We all know that founders are smart, they're passionate, they believe in their dreams, and they feel that they can pull off something which other people can't. They're willing to take on extreme challenges and prove to the world that they can make a difference for the better.
This is a tall order. You need a healthy dose of self-esteem and a strong ego if you want to become a startup founder. This is all for the best, because you need that self-determination and self-belief if you're going to overcome all the hurdles which the world is going to put in your path before you can finally accomplish your goal.