As a product designer, I've learned...
1. You are not your users.
2. Your job is designing for people who know less than you. You know too much.
3. Align users goals and business goals well. Business benefits when the user reaches their goal. Not the other way around..
4. Good design is invisible.
5. Your job is to makes users effective, not just happy.
6. Ask users what they think and feel, and not how to solve the problem.
7. Don't be a design monkey.
8. Ask good questions to get good answers.