Or you know, you could save yourself even more time, not watch his video and not say anything.

You know, I actually like watching good amvs, and I know several people that if they didn't get nudged/encouraged at the right time they might still be making noob videos.
[edit]Besides, I don't give that level of critique unless directly asked. (or prompted to give an example as in this case). I may poke a line and say "You know you can improve here, if you want more detail on that, ask me" and if they ask, then I might go into it. I know better than to waste my time on every single nooby amv that comes up.
But when all you do is tell the editor how forgettable their video is, you aren't helping much or giving a valuable critique either.