My partner and I got a YC interview last round (no luck this time, unfortunately) and one thing I'd like to caution is that if you have a "chicken and egg" situation ("Why would X users use your site until you have Y users and vice versa?) or an "established competitor" situation ("What's to stop Google from just adding a link to X product?"), you should have as close to bullet-proof responses to those questions as possible.

Our interview was pretty much spent trying to respond to those questions. This is why having an actual product with any sort of customers is so valuable, because that's live evidence that what you're doing is working. Unfortunately my partner and I had little more than an idea and half of a web application, which we didn't even get to demo because we were so caught up trying to defend our position. I literally never even turned my laptop around to PG and company, which is probably my biggest regret about the whole thing, because I had spent every free moment of the past several months working on it and maybe that could have said more about our ability to execute than a debate on whether X market for Y users even existed.

Then again, probably not.