We emailed a few YC alum to review our application last month, and they all said we were a great team. One loved the idea, one liked it, and one had no clue about the market (wasn't familiar with Facebook and quizzes). Our idea targets young people (12-25 yrs). Which leads to my thinking, that because they're unfamiliar with it (YC partners too old, would never do facebook quizzes), they brushed it off? One other issue is that they prefer built out products with users. We only had a working demo and proof of concept samples.
Either that or all the other teams were more spectacular. Which bothers me, cause I've been looking people up on twitter who say they got interviews, and their products seem mediocre.