When I review, I like to offer people constructive criticism, weather it be good or bad.
Often times I'll check if the author responded, and while doing so I'll see that my review gets marked as useless(see bad).
I'll also see this for other users, and most of the times it was when they reviewed a bad movie, and they still tried giving constructive criticism.
Please flash artists who this applies to, if you can't handle even constructive criticism, then don't upload onto Newgrounds, take it to Youtube where you can disable the comments.