I am relatively active on sites like deviantART and fanfiction.net, which, for me, means that I spend a reasonable amount of time reviewing/critiquing other people's writing (and sometimes, hoping that people will review/critique mine). While, I wouldn't consider myself an expert in the craft by any means, I am beginning to notice that my critiques fall into two distinct categories.
There are critiques of work I could do myself, or that are similar to work I could do myself, and there are critiques of work that is significantly better than what I can write. The first set are easier, but I think the second are better for me.
In the first set, I see what I could do better in the piece and explain how to do it. Then I look at the aspects that I don't think are good as they stand and comment on those, although not necessarily in that order. There may be parts that are new to me, but on the whole, I'm just explaining something I've already learned and understood.
When I critique a piece that is better than I can produce, I really have to wrap my brain around it. There is a challenge in understanding what exactly makes the piece special. It's sort of like the inverse of the Anna Karenina principle: every excellent piece of writing can be excellent in it's own way. Then I have to look beyond what I like about the piece to what could be improved. I suppose that the more I go on, the easier it will become, but trying to improve on the (presently, by me) unattainable can feel a bit pointless. I usually get there, though, since none of us are perfect.
Going through this whole process can take a while, but at the end I've definitely learned something from it. I think I've given something more valuable to the author, too: if I use a critique to explain punctuation of dialogue or to suggest observing people in real life as a basis for characerisation, I'm not really giving the author something that's uniquely mine or uniquely theirs. If I can find something that touches me in a piece, or I notice an uncommon flaw, then I can tell the author something that might not be relevant to most people and that might not be noticed by everyone.
I'll certainly carry on reviewing all sorts of work, but I'm going to try to remember that the harder critiques to write are also the more rewarding. Falling back on doing what's easy doesn't grow me. Actually, that's probably true of everything I do.