I also interviewed at YC last week but didn't get in. Our product is fairly simple -- we make it easier for anyone to book a service and we're starting w/the massage industry vertical. Here's part of the rejection email we received:
"This wasn’t an easy decision because, as you pointed out, there’s clearly a lot wrong with the systems that people use to book massages. It’s a similar problem to the one in many other appointment based businesses, and solving it would be valuable. Given the nature of the market and competing products, though, we could not quite see how you’d build a defensible product. Additionally, based on the way you’re planning on structuring your fees and relationships, it isn’t clear how you’d be able to lock your customers in for long enough to recoup your expenses and profitably acquire new customers."
We also applied for and/or interviewed at other incubators. What sets YC apart from those other experiences is the great feedback we get from them.