Jack...Lots of good suggestions but it seems to me how you respond (and I, too, think you should respond, one way or the other) depends on how many submissions you get. Giving honest criticism that isn't destroyingly hurtful, can take a lot of time to devise (and may not be appreciated by everyone!). Worth the altruistic effort, maybe, if you've got the time; otherwise, generic letter w/personal line or 2 seems more reasonable.

As a professional studio potter, most of the art shows I did were juried, and required slides. Rejection letters were all over the place, from a simple no thank you, to a few unsophisticated, and very condescending "criticisms." All were taken in stride...all part of the game...and sometimes it IS a game.