The feedback you want is the main value YC offers, they won't provide it to teams that were not accepted.
I think pretty much all teams are great. Perhaps there isn't a very specific reason for rejection once you get past by the alumni filters and is evaluated by PG himself.
That said, I wouldn't be surprised if the alumni are filtering teams based on their own values and reality. I think alumni having any contact at all with the selection process makes it extremely biased and will generate a lot of look-a-like startups.
It's not that they are bad or good, but this is how humans work, bias is natural.
I dare YC to choose 10% of teams at random next time, not even reading their application. I bet the results will be the same, if not, better than the average.