It really comes down to remembering who's behind a production and what they want to accomplish. People who produce videos for a living should be judged on one scale, while amateurs should be judged differently. I certainly don't mean that someone should be told that something that's "bad" should be praised as "good". I have in at least one case passed along some rather pointed criticisms of fan productions in PMs. I don't see anything wrong with choosing to praise effort and enthusiasm publicly whilst saving the more pointed criticisms for private conversations.
On the other hand, if I want to crticize JJ Abrams or the "brains" behind "Nemesis" I'll just let it rip and those on the receiving can cry all the way to the bank (if I care to flatter myself into thinking that they care what I think)