Constructive and USEFUL criticism of any piece of art is hard work. Just saying "Nice work" may make the artist feel good and "What a load of s**t" hurts, but neither type of comment really helps the artist/photographer develop his/her skills. Even brief comments like "Your composition could be better" are not much help unless followed with an explaination as to why the writer thinks the composition is weak AND how it could have been improved.
In many cases it is difficult to comment on a photo unless the exact circumstances under which the shot was made and the photographer's intentions and equipment are known. A shot would be considered very good if taken from the back of a moving pick up truck. The same shot would be considered awful if it was the result of hours of set up and a dozen assistants with reflectors. Was the photographer trying to make use of light and shadow or was it just underexposed? Was he attempting "soft focus" or was it just bad focus?
I think that you will find that in those lucky groups where there is consistent and quality feedback, it is usually a few dedicated and skilled artists who are trying to pass on their hard earned knowledge.
One final point. Many people do not really want comments. They just want compliments.