It surprises me that both the linked post by Guy Morita [1] and this one describe that they not only got detailed feedback, but that in both cases the problem was with the idea. I've always thought the idea was the least important thing, as almost everyone changes their idea, and attributes of the founders such as determination were much more important. Does YC simply limit their feedback to the idea?
[1] http://alchemist.svbtle.com/our-yc-interviews