No one wants to have their images bashed, my motto is:  "If you don't have anything nice to say, don't say anything at all." 

For the most part, practice makes perfect.  I think that people will eventually improve their techniques on their own without "constructive criticism" if they keep at it.  If people are discouraged, they are more likely to give up, and that would be sad because creating digital art can be a very satisfying hobby.