There is a WORLD of difference between concrit, even when it's harsh, and a flame. To me, a flame is name calling, stirring the pot with others to make THEM flame a certain person (flame baiting), and the like.
I've often left what many would consider harsh concrit, because by nature I'm very blunt. So, when I see things and point them out, I'm not going to pussyfoot around and circle talk to get my point across as to the problem I'm seeing. I'll just say what it is and move on.
Honestly, in some for instances it's VERY hard to leave something that's not going to be taken badly, depending upon the person you leave it FOR. People can get very defensive over the silliest things, really.