Rob May at BusinessPundit has given up entrepreneurship (for now) and gone back to a regular pay check.  He shares some of the lessons he’s learned in this fabulous post.  Here are a few of my favorites:

Know how you make money. Ideas are great, and I'm all for doing cool things, but cash is still king. How do you get cash?

Your estimates are wrong - Yes, even your worst case estimates.

You aren't your own boss - Your customers are way more demanding than any corporate boss could ever be.

Nobody cares that you are smart or knowledgeable (and you need to know if you really are) - Why not? Because everyone thinks they are smart and knowledgeable. Everyone is convinced they are good at business. Everyone thinks they can hire a talented team. Everyone thinks they can sell. Everyone thinks there is something special and different about them that will make them successful. But accruate self-evaluation skills are critical to entrepreneurial success. You have to know what you are good at, what you aren't good at but can learn, and what you will probably never be good at. I think this is a major reason businesses fail.