i also agree with the notion that a bad response is better than no response.
there have been times i have posted photos and gotten nothing as a reply.
this means that either my work is so good that people are dumbstruck and cannot type anything
or that its simply uninteresting (to them)
i'm going to lean towards the latter one...
i think if you have the will power to comment negativly on a photo, all the power to you, especially if you can put that in the form of constructive criticism and explain your logic.
for obvious reasons saying "this sucks, go hang yourself" would be inappropriate, but i haven't seen any of that.
regardless however, anyone posting on a public forum should not take offence to public criticism even if it wasn't asked for. You dont walk in public wearing nothing but your underwear and expect for everyone to keep their mouth shut.