Paul Graham, an essayist, programmer and programming language designer, just wrote a very interesting essay on the hardest lesson for startups to learn. You can read the entire essay here.

The key lessons are:
1. Release early
2. Keep pumping out features
3. Make users happy
4. Fear the right things
5. Commitment is a self-fulfilling prophecy
6. There is always room
7. Don’t get your hopes up
8. Speed, not money

Enjoy!