I'm notoriously bad at getting concrit, and I'm not much better at giving it. I don't like to hurt people's feelings, and I don't trust my own opinions. When I do give concrit, I tend to point out any spelling/grammar errors and typos, and to pick out a few phrases that I really liked, or a little element that I loved.

In future, if there's any element that didn't do anything for me, I might try and mention that too.

But I was thinking about concrit, and me, and the way that my English teachers could give me back a story absolutely covered in red pen, and I would grin and I would edit. Sometimes I didn't agree with what they said, but I'd look at their suggestions and rework what I'd written a little -- not so that it became more their story than mine, but so that what I wanted to happen became more believable for them.

On the other hand, someone left me a comment picking on the structure of the piece, the characterisation, etc, and I hated it, because they were stating as facts that these things didn't work for them so they didn't work for anybody.

I was thinking about the different critical approaches I learnt for English Lit., and I thought of one I think is really appropriate when it comes to fanfiction: reader response. We may not all be English teachers, or particularly literate, or even native speakers, but we have our own responses to something. Often, that leads to someone saying "[plot point] is wrong because [character] wouldn't do that". Sometimes it's obvious that that's just their opinion, sometimes it comes off as "hey, everyone who reads this thinks you suck!".

I think that the easiest way to review and dodge offending anybody, save the least willing to listen, is to pick out the thing that forced you out of the story and say "[plot point] doesn't feel right to me because the impression I got of [character] in canon is [x] because of [canon plot point]". Or perhaps "I'm not sure why, but [plot point] doesn't feel right to me, perhaps you could tweak it so that [x] instead" -- that kind of thing. It sounds like a lot of pussyfooting around, but I think that I at least would take concrit like that a lot easier. Obviously, grammar, spelling and such aren't subjective, but for anything that's a matter of preference, it's probably best to make sure it's obvious that you're entering the world of opinion -- to think of it not as "concrit" but as "reader response".

Of course, there are some people who don't do that and leave brilliant responses that help the authors. There are authors who won't take even that and will stir up a shitstorm about it, and "opinions" are often more volatile than cold hard facts -- but opinions disguised as "cold hard facts" are worse.