On one of the forums I run, we have writing and artwork forums. The same thing happens. No one will give constructive feedback, even though the staff has constantly said that giving negitive but constructive feedback is okay. I don't think it has much to do with people being moderated for negitive feedback, but more to do with people not wanting to seem like the downer or a negitive person. If 3 people before someone give all smilies and "thats gold" comments, then it is doubtful that the next person in line will leave negitive feedback.
Just my observation on the matter.
