In no peculiar order.
1.The value is in the team, not the product. An awesome team will eventually become successful even with an average idea. A poor team will likely fail, even if their idea is amazing. Many startup founders fail to realize this.
2.Progress is not linear. On a day to day basis, the time you put in does not equal the results you get. In fact, results come in at random, in huge peaks. Most startups quit or run out of money just before they get to see that happen.
3.The biggest threat to a startup is the startup itself. Most companies implode or explode due to internal issues, not the competition or the market. Organization, communication and team alignment are more important than you think.
4.Nobody really knows what they're doing and that's ok. Being an entrepreneur is not about knowing more than the next guy. It's about having the courage to do something with the information you already have and hope for the best.
5.Action trumps planning. You can't plan your way to success. It's ok to think about things for a while, but elaborate plans are a waste of time. You can't learn by planning, only by doing.