What might be useful are comments overall like "we received several applications for online invitation services but nothing seemed innovative or compelling enough to us" or "we received a handful of applications for doing x but, in our opinion, the mobile space is where we think that market is headed". I think you once mentioned that you get a ton of hotel/food review website ideas. You've also indicated that you think the email overload management is an area of YC's interest.

Doing this might help at least some of the applicants get some idea. Not everyone is covered by these comments and no one is forced to take your opinion for it but it just helps to get broad feedback.