Here's the thing. Exchange fic or not, I see all fanfic as being "gifts" in one way or another. We're not getting paid for this, and if we look hard enough, we're finding stuff that might as well be written for us. On the other hand, a lot of stuff? Is very definitely not written for us. Fandom is like a huge, enormous, office Christmas party in which people sometimes get awesome gifts, sometimes get awful gifts, and sometimes their gift-givers are mysteriously absent. At that point, you have to get into the question of whether it's rude to leave critical feedback at all.
And that depends on what you mean by "leaving" feedback. Are we talking about wandering up to the author, poking them, and saying, "Hey, this and this and this didn't work for me. And Clark Kent? WOULD NOT EAT MARGARINE."? Because no, I wouldn't do that. But if I'm writing reviews in my journal of stuff I've read, I may launch into a five-line rant about Clark eating margarine, because I'm in fandom to talk about my thoughts on characters I love (or... sometimes just watch, as I don't love everyone. But you know what I mean). I want to hear what other people think of Clark eating margarine. Do they agree with me that a Kansas-raised guy who grew up on a dairy farm, is immune to the problems presented by fat and cholestorol, and can soften the butter with a mere look, will have no use for an inferior bagel spread? Or do they think he'll give in to his current girlfriend and suffer with her taste for vegetable-oil-based fake butter?
When it comes to gift fic written specifically for me, I will leave positive comments no matter what I really think of the fic. There is always something to praise, even if it is merely the fact that this person took a request of mine and wrote something for it. When it comes to negative comments, period, I won't leave those on a fic, but I have absolutely no problem writing about them in my own journal. I hope that's a distinction people are willing to make, because suggesting we can't discuss the things that left us cold about certain fics even in our own journals takes niceness way too far. Reviews should not be 100% praise all the time; that takes away part of their utility (think about this for a minute: someone with completely divergent taste from you writes a review and hates a fic. Everything she hates, you've always loved. She hates something you've never heard of. Worth a whirl? Maybe?), as well as putting the kibosh on fans' ability to discuss things with other fans.
Here via metafandom
And that depends on what you mean by "leaving" feedback. Are we talking about wandering up to the author, poking them, and saying, "Hey, this and this and this didn't work for me. And Clark Kent? WOULD NOT EAT MARGARINE."? Because no, I wouldn't do that. But if I'm writing reviews in my journal of stuff I've read, I may launch into a five-line rant about Clark eating margarine, because I'm in fandom to talk about my thoughts on characters I love (or... sometimes just watch, as I don't love everyone. But you know what I mean). I want to hear what other people think of Clark eating margarine. Do they agree with me that a Kansas-raised guy who grew up on a dairy farm, is immune to the problems presented by fat and cholestorol, and can soften the butter with a mere look, will have no use for an inferior bagel spread? Or do they think he'll give in to his current girlfriend and suffer with her taste for vegetable-oil-based fake butter?
When it comes to gift fic written specifically for me, I will leave positive comments no matter what I really think of the fic. There is always something to praise, even if it is merely the fact that this person took a request of mine and wrote something for it. When it comes to negative comments, period, I won't leave those on a fic, but I have absolutely no problem writing about them in my own journal. I hope that's a distinction people are willing to make, because suggesting we can't discuss the things that left us cold about certain fics even in our own journals takes niceness way too far. Reviews should not be 100% praise all the time; that takes away part of their utility (think about this for a minute: someone with completely divergent taste from you writes a review and hates a fic. Everything she hates, you've always loved. She hates something you've never heard of. Worth a whirl? Maybe?), as well as putting the kibosh on fans' ability to discuss things with other fans.
Not everything is about the author.