I hate not being able to give people gushing reviews of awesomeness. I want nothing more than to finish a story and only be capable of giving a one-word review: Brilliant. I want to be swept away in a cascading seduction of a beautifully written story. I want Aristotle to be right: Plot. Character. Theme. Diction. (music) Spectacle.
A good story well told.
It's a lot to ask for.
Most people can get even a basic plot down. Since AFF is mostly 'adult stories' and by that it sort of (unfortunately) translates to sex and porn, plot tends to get slightly forgotten... Which is a shame because that leaves Goggle to find something with plot, written with intelligence for us Yaoi writers since FF.net decided to oust us. We didn't move over here like I'd thought people would. Which leaves a lot of PWP which is fine if that's what you are looking for. Gods above know I've hunted for good porn more than once but really, I became a writer because it was another outlet for me: I'm a storyteller. It's GOT to be a good story. You can't have a story without plot.
Characterization in fanfic is a mercurial thing. It can be very hard to agree with characterizations just for the sake of a kink or the advancement of an unlikely (or AU) plot. I write AU with Zalman's Heart and it's a disheartening thought to know someone is going to complain that our characters are COMPLETELY OOC and we stole from some 50+ different shows characters everyone loves and we had to have them. But we did that knowingly. We even warned people shit was different. But character interpretation is a hard thing to argue. 'Cause once you write them they are no longer the author's character, they are your character and hell be damned no body gets to touch them. But if you are going to go with radical changes I would request an explanation for the change from the show's character to your character somewhere. Your audience is reading the story for the show, characters, or the interesting summary tag. (Did I miss any? Sex, maybe?) The changes shouldn't make me want to withdraw my willing suspension of disbelief. I'm a yaoi fanfic writer, I have a lot of suspension capacity.
Themes: have one. Please. Somewhere. It's a through-line, it's the bow all the threads at the end of the story get tied up in. It doesn't have to be AMAZINGLY complicated. Teamwork. Love. Hate. Pain. Good guys should always win. Good guys aren't as good as we want them to be. Bad guys can be good guys. I don't care what it is, so long as you the author can pinpoint what the theme is if I have to ask.
Diction is my biggest pet peeve and the hardest to talk to writers about. A lot of it has to do with age, maturity, education, and style. Young writers tend (operative word) to not have the skill to convey a captivating story. Practice does make perfect only if the feedback helps him/her mature. If you can't read something because it so poorly written it won't matter the amazing-ness of the plot or characterization. Education is a huge factor. Younger writers sometimes just haven't been given the information, the structure, why the structure is as it is, and how to use language. I started writing fanfic at 14, I read a lot, I paid attention, but until college my writing was immature because I didn't understand the structure, why, and how to break it to tell the story better. Until college I didn't timeline, research, or pound out plot charts because I didn't know I had to. How do you explain to a 14 yr old that you need specifics? Their story wasn't exact enough? It had plot holes or the time-line didn't work? That their description (if they have those) of the Corinthian column is actually a Roman composite? Try convincing some 14 year old kid to do their homework on what they are writing about. Or they'll just dump excuses on style. Style should elevate the story, not disconnect the readers. Maturity is know the difference in all these things and others I won't get into. It's that personal strength to know what matters to your story most and how to best showcase it. It's the ability to have someone ask questions in the margins and being able to give them answers even if it's, "I don't know, I'll have to look it up/decide." I always want more description; my mind needs that moving picture. Unless the person has that genius style to describe a setting in eight words that give me the tone and mood and time and place. But that in itself is a mature style of a highly educated person. That precision word choice is a kind of dedication that can't be asked of most adults, never mind teenagers.
Music doesn't apply much to fanfic, except in song fics or fics inspired by songs. The later is easier to deal with than the former because it is almost impossible to read the entire fic at the pace the song plays. Poets, on the other hand, can make music of spoken word.
Spectacle for me is mostly the sex. Generally a story doesn't need sex, it's just window dressing to draw people in, I think. A small hook that hopefully the plot will override as the reader continues on.
When I get a review that is negative I go over my goals I had set for my story. Did I meet these goals? Are they pointing out a plot problem? A characterization screw-up? Was my writing style boring? Too descriptive? Did it not have enough sex? What was the point? Did I make that point? Is this thing the reviewer didn't like something I could work on? As a writer, what do I personally think I need to develop? Or has my ego grown so big that I don't think I need to work on anything and that I'm PERFECT? Do I think my story meet all the Aristotelian requirements for a good story well told? I once had someone leave a review that they didn't like the kind of magic system I had used for a DNAngel story. That was all s/he said. There was no hint on what s/he thought would have made for a better system to use, or what could have been altered to heighten the tension or keep the tension and change the aspects s/he didn't like. I was more angry that it was a useless one-line than it was negative. The only thing I could think of was that in his/her religion, ritual magic is used and my corruption of it for the story insulted them. A haphazard guess at most; I only had one line to work with.
It's hard dealing with insults and negative comments but the maturity of a writer--or indeed anyone--is to grasp the root of what is being said of their skill and use that to overcome something that might not have even been in your consciousness before. That's one good thing about having a beta that works on a different plain then you; they will see and know things you would not even consider.