What do you wish you had known before starting up ?
edw519: Great question.  Off the top of my head...  1. if (ethics != 100%) return(DontBother);
  2. Regardless of paperwork, you're *married* to your co-founder.
  3. if (CofounderMarriage == wrong) return(StopNow);
  4. Customer first, technology second
  5. Make your customers love you, everything else gets easier.
  6. Evangelize, evangelize, evangelize.  People love passion.
  7. When in doubt, release. If you did #5, they'll understand.
  8. Get constant feedback. You don't know your requirements.
  9. Run out of cash and you're dead.
  10. Have fun!

[EDIT 11. Accept feedback willingly and implement as appropriate.  Thanks, idlewords.]