Earlier quoted context omitted.
if we are not even given any feedback as to why our applications were rejected, then it seems peremptory and a complete waste of time. Surely, they could have copy-pasted even one summary line from their evaluation sheets? It is a simple courtesy.
Agreed, something like "think more about tech question", or "think harder about innovative idea" would have taken 10 seconds, but have been significantly more valuable to an applicant - and yes, it is courteous to do favors for someone when the effort you put into it is so dwarfed by the benefit they receive.
And I think being too specific could make matters worse. Think about dating. "You are a nice person, but I don't really like your nose" - helpful? I think not, and the next person might be just fine with your nose. I think the generic letter YC sent (I got one, too) is a good solution. And I could imagine that if you would ask them, they might be willing to give you more specific information.