I like this post; it's really apt. I just read a fic by obsession_inc, called Concession, and I was really blown away by that story (which analysed and deconstructed gender roles in the context of stalking, normative 'romance', and abuse). So I commented on those aspects which I found striking. That's typically what I do if I really, really like a story - I talk about what I like, what I felt worked. I'm not going to do a full-on edit, but if some aspects don't work or could have been fleshed out then I try to include that in my feedback as well.

I don't like to comment on a story if it sucked. If it had its strong and weak points then of course I'll try to give a more balanced viewpoint, but unless it's something that's really awful (like, maybe, rape apologism or Holocaust denial or exotic orientalism, something godawful and offensive) then I try not to leave negative comments.

Similarly, I like people to take a similar approach if they comment on what I write. I don't mind someone writing with their personal feelings; I try to respond in kind; but sometimes if I'm at a loss for words then I'll stick to a 'Thank you' or 'Thank you for reading/commenting' and a passive-aggressive :) emoticon.