"If you don't like it, that's what your Back button is for" school of thought.
I think it's an awesome school of thought -- I subscribe to it most of the time, in that I hit the back button without finishing reading if I encounter enough stuff that makes me dislike the story -- so I don't actually reach the comment stage. But once someone has sat through the entire fic, I think they're entitled to their thoughts and to expressing them. Unless the author has specifically requested, upfront, that people save the crit for someone else.

I would have liked a little less Luna, but your Draco was particularly lovely.
Is this the kind of comment that people are considering "concrit" these days? If so, I can sort of understand what all the frothing is about, but this isn't concrit. This isn't even criticism, really -- it's someone sharing what they would have liked to see in the story. If an author takes actual offence to this, I think she's the one in need of some sensitivity training, because any author who gets offended or upset at someone passingly mentioning their dislike of something comes off like an egomaniac who expects people to talk about her, her, her all the time. People like to talk about themselves! This is not news! "I would have liked a little less Luna" =/= "I am disappointed that you wrote so much Luna." I agree that it's not constructive criticism, but it is an opinion that the reader is entitled to hold and to express -- unless, again, the author has specifically requested no criticism. I think I'm going to make a new rule for myself: read no fic that doesn't specifically indicate that concrit is welcome. :D