1) The YC staff and Alumni possess a wealth of knowledge that would be available to you.

2) To get out of your bubble. Being surrounded by other intelligent individuals all working toward demo day helps you stay focused and keep moving.

3) The experience. Being able to have gone through the YC system wouldn't hurt.

4) The seed money would be a huge help.

5) Access to VC's with the YC brand name behind you. So again, the money.

6) The alumni network. I hear it is extremely supportive even after YC.

Disclaimer: I applied to Summer '11