Thank you so much for the link, very helpful and I've actually read through PG's blog posts and advice before. But I couldn't isolate any indication of progress prior to application. ie. do I have to have traction prior to application? Does this have to be in the form of users or can it be in the form of registered interest?
Thanks!
PG's most consistent advice is the founders are important, not the idea. In theory, the right set of founders could apply with no product and no idea and still get into YC.
Likewise, PG recently said that applying every six months with a different idea makes your likelihood of getting into YC better versus consistently applying with the same idea ( http://news.ycombinator.com/item?id=5194256 ).