Also: this is not a lament about the quality of FFN fic, reviews, or anything like that. It's something that occurred to me when I was laid up with foot problems last night.
The issue I'm thinking about here is what you might call "good writing" versus "bad writing," which are pretty subjective terms. According to my writing program, literary fiction is good and genre fiction is bad; according to fandom elitists, fan fiction with a well-structured plot is good and fluff is considered bad, although in general, I think most people will judge based on the elegance of your sentence structure and whether or not they get bored.
In a perfect world, the quality of one's fiction would determine how popular it is, how many awards it wins, and how many reviews it gets in the case of fan fiction. The professional world is probably better about this (i.e. you won't see a shitty story win a Hugo), but we've all come across books that make us question how on earth the author ever got published. Fan fiction is even worse because there aren't any filters (editors - with the assumption that their taste is correct) between us and the fic being displayed in the archives these days. But this doesn't seem to be a problem - "bad" stories get hundreds of (positive!) reviews all the time, so clearly someone is reading and enjoying them. Likewise, quality fiction gets ignored on a daily basis. This boggles me in both contexts, but has been especially irritating over the years in the fan fiction community, where one is expected to improve oneself, but not necessarily rewarded with success for doing so.
So look, it has always floored me that, for example (I'm pulling this out of my ass), a fangirly Naesala story can get two hundred reviews in spite of the rudimentary skill evident in the writing. The author might be able to spell, maybe she has a fair grasp of grammar, but her writing consists of cliche after cliche - Naesala is always running his hand through his hair (because that's what he did in that PoR cutscene, and it's hot!) and Leanne is always fluttering her pretty heron wings and speaking in broken sentences, and they're madly in love because the author tells us something like, "he loved her so much he couldn't sleep," or another better cliche I'm not thinking of, because it's hot and I'm feeling brain-dead.
To me, that's awful writing. To most Naesala fangirls, the triggering phrase Naesala ran a hand through his hair and grinned inspires instant squealing and optional panty-creaming because that gesture is perfectly in tune with the game - in fact, it was in the game. No thought or imagination necessary. Meanwhile, another Naesala fan is over there ripping her hair out over the perfect way to depict his sexy arrogance without rehashing what is essentially a script point, and the response to her effort is quieter, if it's there at all.
So-called "crappy" fan fiction is so popular precisely because it replicates something in the reader's original experience of canon - a real image, in Fire Emblem's case, or maybe something reminiscent of a sentence or passage from the original book, and so on. In fan fiction that can work because we have those existing images in our minds; in context it might not equal bad writing, because in fan fiction your ultimate responsibility is to canon; even if you want to write an AU, you're starting with an authentic canon character, and trying to decide how that character would change from canon in a steampunk setting, or a high school setting. If you're trying to fit your fic into existing events, you're even more a slave to the original work. And who is to say that, when Naesala rescues Leanne from the Twisted Tower in FE9, he isn't making his trademark hand-through-hair gesture, because that's just what he does, and even the developers would use that as characterization shorthand if the character models were more elaborate?
That doesn't make it good writing, but might make it good fan fiction according to a general fandom consensus - as much as a fandom can agree on such a thing, anyway. Like I said above, quality is subjective to an extent. Genre - or lack of - does not determine quality. Neither does entertainment, but the latter is how many people I know determine whether a movie or book is "good" or not. And to tell the truth, I'm willing to fall for it myself, if it means I'll get to see work for a character or pairing that rarely gets fic.
I would say this isn't true of professional fiction, but isn't it? Not all genre fiction is bad, but I can open a Dragonlance novel, for example, and find plenty of passages that tell me how characters are feeling instead of showing how and why. Bad writing, according to my teachers - and yet it's functional, and millions of readers out there don't care, and will sympathize as much with that example as with a well-executed depiction of his disappointment. And I suppose there's a time and a place for everything, including telling-not-showing. You can't depict everything in super-specific detail. Your readers will get tired.
However, thinking about it this way tempts me to resort to stupid cliches. I won't, because writing something like "I don't know empress," Naesala said sardonically, hand running through his hair. "How much am I worth to you?" would make me cry, but attempting to do better all the time is awfully unsatisfying when it doesn't also result in better response. And with that in mind, I think the attitude in fan circles that all writers should listen to critique and better themselves is kind of hollow, because expectations set the bar low, and most fanfic writers want attention and people to fangirl with, while most readers are satisfied with setting the bar mid-level at best. Quality appears not to be the point-- at all. Rather, the point seems to be replication of the original material ad nauseum.