We surveyed the founders after the last batch. Their main gripes were: 1. There aren't enough social events, especially early in the cycle. So we're adding more. 2. Founders were surprised how harsh our feedback was at times. We may be able to be more diplomatic. But I think the fundamental problem is that this is a domain where, so far at least, many participants fail. So if you're truthful, you're often going to be…
Disclaimer: I don't have YC experience.
In school, my classmates and I too often experienced the opposite kind of feedback: pat on the back, great job, I thought it was fantastic, etc. What little criticism was given usually came in the format of a "compliment sandwich."
Although praise is nice, in general I find that being nice to avoid confrontation or to prevent hurt feelings only promotes complacency and lack of (personal) growth. Not to mention if you tell me something was great when I know it sucks, you've lost all credibility with me.
Don't sugar coat it - I have a pretty good general idea of what's working. I find honest feedback more helpful.