Hopefully I won't feel stupid posting this, but I wished someone had done the same thing when I applied. (pg -- if you'd rather I not share this, feel free to delete)
Even though I don't intend on reapplying, this was thought provoking; thanks.
Why would your project be hard for someone else to duplicate? I think this is a critical question.
Several weeks ago I was at a coffee shop and some girl was complaining to her friend about saving her data as they were leaving. Her friend said, "I just use DropBox." Had to smile as it was the first time I saw a (presumably) lay person mention a YC-funded company. It was nice to see an indication of such a breadth of success.
You had applied earlier (SFP05?) and been rejected, right? If you don't mind, it'd be really interesting to see the earlier app too and compare what changed between them?
You seem to be already doing well at the time of the application, why were you applying to YCombinator? Was it the money, the networking, or something else?
The document was very informative. A repository of YC apps would definitely be a tremendously useful collection, not just for future YC applications, but for general startup community as well.
Original Dropbox YC app: http://files.getdropbox.com/u/2/app.html
Had to remove some names and one sensitive thing but otherwise unedited