We just submitted our application and I have to admit that YC's application questions themselves are a great exercise for anyone who is starting a new business. I felt that all the questions really made us think about each and very aspect of our platform and challenged us to articulate it in simple words. We are excited to see what comes out of it and good luck to the rest of awesome ideas!
I couldn't agree more. Filling it out the first time we applied actually led us to the full developement of our business plan, right down to the design of the GUI and the financial metrics we would use. We now know exactly what is required of us to become profitable, how much we can stretch our margins, etc etc. Granted this is not necessary for YC but it let us know exactly where we stand and how hard we have to work before we can all afford to eat again.
The application process also taught our founders a little more about eachother. The things my cofounders put as their "most impressive acheivements" blew me away.