-Unnecessary lyrics: I understand songfics on some level, as well as the placement of lyrics in parts of stories that aren't, but sometimes it's just fucking insane. The character (usually either an OC or an OOC canon character) is lying on her bed, listening to music, brooding, and each angsty, generic paragraph is book-ended by a few bars from Nine Inch Nails songs, and this continues throughout the story in every scene that music is played. Having those lyrics there just reminds me there is something better I could be reading that isn't a Mary Sue's insights to how the song really speaks to her soul in a scene that happens with no consequence other than being heathercullen666's mix CD.
-"My writing really isn't that good and I don't know why I wrote this and I probably suck": Good, then I'll skip over to the next story. But hey, thanks for putting it in the author's note so I didn't even have to +1 your view count.
I can understand submitting something you don't think is up to your standards. I've written things that I felt were good, but not entirely up to what I hold myself to, but I still posted them. On the other hand, I also write stories that I can look at and say "One of my best. This is a pretty good fic". I'm critical of my writing, and it honestly surprises me sometimes when I get a glowing review for something I felt weak by my own standards, but I'm not going to mope, I'm going to take it as a compliment. If I submit a one-shot based on half a plot bunny and it fizzles out somewhere down the line, but I still finish it, I'm going to feel much better about my writing to know that even something I feel isn't up to my standards is still well-recieved. That's a testament to my writing, not unwarranted feedback I don't deserve.
-Three page author's notes: When a fic gets long and has a lot of backing behind it, particularly when there's a set of people who regularly leave a review after each update, it is a good idea to maybe address questions posed or provide some information about the story. Keep it quick, though.
There's no need to go on for what seems like half the fic talking about what's going on in your life that kept you from updating. You don't need to respond to every review, either, commenting on each one. Thank them in one big group by saying "Thanks to everyone who left a review since the last chapter" and repond to the actual questions.
-Not even using spellcheck: With open-source word processors like OpenOffice available for free, there's just no damn excuse for having a story rife with obvious spelling errors, not even ones that people make by omitting an l or such, but things like "taht" and "wnet". Even if you don't have a program that can do it, at last fix what you can.
-Unlabelled crackfests: I don't just mean crack pairings; I should be able to tell it's a Snape/Cho Chang fic and be able to stay away from it accordingly, I mean stories designed to be stupid, out of character, and probably intentionally bad, but that don't indicate it in the description or even the author's note at the beginning.
-Poor descriptions: This ties into the above. A half-decent writer should be able to put in, from the description, the general type of story; if it's romance, smut, crack, or the like, it's not very difficult to write a description accordingly. Here's some half-assed examples:
"Harry's the victim of a broken heart, but will he find solace in Hermione's open arms?"
"Harry and Hermione find some time for a quickie in the common room at Christmas."
"Hermione's got some weed, and Harry's up for some puffs."
Guess which one is smut, romance, and crack.
All I'm saying is, I want an idea of what kind of story I'm about to click on. Content tags only go as far as describing the sex, but I may want to find something fluffy that will give me diabetes.
-Script format: Unless it's a transcript of two characters talking over MSN, the layout should not be the character's name, a colon, and what they said. Even if you aren't describing actions, at least just use quotation marks. If there are two people, you don't need to say who is talking every time, as we can gather that information by the fact there's two people talking one after the other.
-Writing characters you don't know: It's one thing to write somebody out of character or not totally get their nuances, but I find it absurd when people write stories in fandoms they don't even know, having just seen two characters in a picture, reading their character pages on Wikipedia, and then writing a fanfic about them where everything feels so horribly wrong. It delves into the downright unbearable when told from a first-person perspective, with no indication of AU or OOC, and everything just feels so unnatural for the character. If you don't know any of the characters, why are you writing something in this fandom? Even at my smuttiest, I write only about characters I'm somehow endeared to.
-AU without realizing the repurcussions: WHen you radically change a universe, you have to be aware of the repurcussions. I remember one a Harry Potter fic where Lilly married Snape and had never met James, and yet Harry still looked the same as he did in the actual HP universe. It's just overwhelmingly lazy writing.