It's hard (or impossible) to give detailed feedback to every applicant. But maybe telling them roughly in which bucket they fell ("shortlist", "pretty good", "okay", "stopped reading after 30s") would be useful.
Yes, it would be useful, but YC's lawyers probably advised against it. Doing this could open the door for a rejected applicant to allege some flavor of unlawful discrimination (pick one: race / ethnicity / gender / age / etc.). I'm not sure the anti-discrimination laws would even apply to a situation like YC, but at a minimum the allegation could turn into a bandwidth-sucking distraction and PR headache.
(disclosure: I work for MC)
I would find it useful to know how I'm doing within the pool. Even if the answer is "bottom 20%", that lets me know how much more effort I have to put into it to improve.
We had concerns about: a) idea b) team c) small market size d) something else