Most pessimists aren't being overtly un-constructive. But there are some very common species of negative comments that are clearly that: for instance, glib or sarcastic dismissals; and uncivil, hostile, or accusative criticisms.
True, those are a good target. I'm thinking of the more dangerous types though, where the person genuinely thinks that their negativity is highly valuable, when it isn't. Those are everywhere on the "Show HN" threads. Elsewhere too. The Internet loves to hate.
Hmm, seems like I'm being all talk. I'll just put my demo online, within the next couple weeks.