The only way I've found to get constructive critisism is to specifically ask for it.  If I want, as an example, to get comments on how to improve lighting within a particular scene I'll specifically ask if anyone has a suggestion or recomendation about improving the lighting.  That way no one feels put off by making a suggestion.  I've also found that doing that it's usually only the people with enough expertise to be a real help that respond to the request.  Others continue to post the usual sort of comments.