First of all, a lot of people have talked about an adversarial interviewing style. Our experience was the opposite. Everyone was super-friendly the whole time. David's theory is that if you only have an idea or a demo, they push you during the interview to see how you can perform under pressure. On the other hand, we had a working website that had been out there for a month, so we'd already proven we could build something. I find it plausible; I think in addition, maybe the more they like you, the more they push you. It would be good to know what affects the interview style, so that you can know ahead of time what you can expect.
In any case, I just don't get why a lot of interviewees were nervous. The waiting room makes you feel very comfortable, and whatever little nervousness you have should evaporate once Jessica makes her appearance and greets you with her radiant smile. I guess for me there were a couple of more reasons why I wasn't nervous: we weren't dependent on YC; we're pushing ahead with the site regardless, and also, my day job involves often speaking in front of 300 people. Sitting in front of 4 people is nothing by comparison :-)
I flew in from Austin. Near the end of the interview, Jessica said, "so do I make the check out to you?" I said yes. "Did you actually spend 700?" I said I'd spent most of it. I actually spent more than 700, but I only had receipts for slightly less. So she wrote a check for 700 right there on the spot. I was kinda speechless.. I was expecting a bunch of paperwork. We kept laughing about it later. That little incident tells you a lot about why YC is awesome. No bullshit.
We thought the interview went really well. But looking at our notes later, we realized we might not have conveyed fundamental things about the site. For instance, they kept asking us if our featured channels were manually curated. We thought we'd said pretty prominently in our application that we do automated, instantaneous channel creation. So guys, remember that they interview a bazillion companies all at once, so don't assume they remember what you said in your application.
Do mock interviews with as many people as possible. We did a bunch, but only one was a YC alumnus. That interview predicted their reactions far better than the others. Remember this. Each person/group has their own world-view.
One last thing: people who got in got the call as early as 6pm or as late as 11:30 pm. We got emailed around 9pm. So stay by that phone.
So that's pretty much it.. our experience was great, even though we didn't get it, and the preparation for the interview taught us a lot, and spawned a bunch of new directions and monetization ideas. Good luck to today's and future interviewees. You'll hear more about WatchUWant.tv soon :-)