1. Fear of failure. Too many people are afraid to ship too soon. They fear a bad product out the gate, rather than getting out the gate in the first place.
2. Lack of focus. You'll spend too much time focusing on things that don't really matter, such as building the perfect messaging system rather than reusing something that exists and works now.
3. Busywork. You'll spend your time doing things that really don't matter, such as optimizing your icons to use fonts instead of a PNG, and then dealing with trying to make it work in older versions of IE.