I think it will be a while before we come to the point where a guideline against "gratuitous positivity" is required. This forum is pretty far towards the other end of the spectrum right now.

I see where you're coming from though. The comments on HN, while often being negative, and almost entirely constructive. I try reading the comments at product hunt from time to time, and while all the "beautifully designed" and "i've been using this since [earlier than you] and i love it" comments are nice and surely make the developers feel good they inevitably add nothing of value to the submission, and as far as providing useful context to the submitted link they may as well not exist.