I actually try not to comment on images that either leave me indifferent or make me think "that's poor".
(Very occasionally I will ask "what am I missing"...)
I wonder if I'm alone.
I know what constructive critique is, but I'm reluctant to try to offer it unless it's explicitly begged for--lest the artist think I'm not being constructive. Faceless words on the internet can so often be misinterpreted. In most cases, I'm really not into hurting an artist's feelings, and never unwittingly.