Paul Graham documents 18 mistakes that kill startups. I particularly like this one:
5. Obstinacy
In some fields the way to succeed is to have a vision of what you want to achieve, and to hold true to it no matter what setbacks you encounter. Starting startups is not one of them. The stick-to-your-vision approach works for something like winning an Olympic gold medal, where the problem is well-defined. Startups are more like science, where you need to follow the trail wherever it leads.
So don't get too attached to your original plan, because it's probably wrong. Most successful startups end up doing something different than they originally intended—often so different that it doesn't even seem like the same company. You have to be prepared to see the better idea when it arrives. And the hardest part of that is often discarding your old idea.