I'm too nice. If I read a fic that turns out crappy, I don't leave a review at all, but I will rate it down. I have pointed out grammar on a few, but in the most nice possible way I can, hell, even I still mess up on my own fics and I'll go back and fix the problem. Personally, I would rather have someone point out a mistake so I can fix it. I don't mind getting critical reviews. It helps me improve as far as I'm concerned. Some people just take it too personal I think.
I am a big hater of flames though, which I have mentioned before on other topics. Critical and flaming are two completely different things. I've nearly flamed an author myself for writing pointless stories that were totally gross and made no sense. All the character were way off, so I just stopped reading. I was going to report the author to the admins, because honestly I think the author gets off on all the flames they get. In the end, I didn't want to stir the shit.
As for the flames in general, I've only gotten two out of the 15 stories I've written so far and I deleted them. I have that right, but I'm not about to delete a critical review that is honest and has integrity.