If the author asks specifically for the readers' opinions, I think people can give them, but it's probably best to give them in a personal message, and not for everyone to read. And I think exchange fests are not really the best place to give criticism. A while ago someone left a comment to an exchange fic that went along the lines of: 'some elements of this story are actually promising.' That struck me as very, very rude, and uncalled for. Does the fact that it's anonymous mean that it's suddenly a free for all? Fortunately most people are not like that.