Anonymous feedback + internet seems ripe for ugliness - see http://hardnewsinc.blogs.com/.a/6a00d83428f05153ef00e553e58b...
Also, what kind of feedback are you expecting people to give which 1)is meaningful to the person it is directed to and 2) does not identify the person giving the feedback. It's very difficult to give feedback which meets both 1 and 2.