Something that prevails on many forums, and photography forums in particular, is the 'mutual back-patting culture', whereby comments like "fantastic image" are commonplace, irrespective whether an image fits the comment.
In the other direction, comments like "it does nothing for me", are equally valueless, unless accompanied by an explanation.
On my own forum (Astro-Imaging), one of the paragraphs in the section seen by 'guest' visitors reads:
"We specialise in the astro imaging aspects of amateur astronomy, and here you will find meaningful comments, along with friendly constructive advice, to help you improve your skills and knowledge of astro-imaging."
We actively encourage critique, accompanied by constructive comments, from which both the owner of the image, and other members can benefit.
Another source of meaningless comments, can come from the 'post count farmers', who thrive on being able to post "Hey, I've just reached my 'X' thousandth post". Look back at their posts and you will find the bulk of them are of the "nice image", or "doesn't do anything for me" type, without any reasoning to accompany them.
'Post count farming' is encouraged on forums where a 'ranking structure' is used, such as 'beginner', 'full member', 'super member', or 'poster god' etc. Some people will post away just to achieve a high rank on the forum, even though the majority of their posts convey little if any value.
If I post an image here or anywhere else, I welcome constructive criticism, as part of the on-going 'learning curve'.
Dave