I have a few peeves that vex me on a regular basis. Some have already been mentioned. Some I haven't seen mentioned here yet.
1 - A fic that claims it is AU and then promptly throws out everything that makes that setting identifiable. To use the Buffy the Vampire Slayer fandom for an example, something proclaiming "AU - Everybody's human! Popular cheerleader Buffy Summers finds herself torn between her boyfriend Xander and the newly arrived British Bad-boy Spike..." or "AU set in the old West..." umm. no. If you remove the reality of vampires, remove the magic, and remove the Slayer-ness, you no longer have the BtVS setting. Period. Write about some other group of high school students. An acceptable sort of AU would be that Buffy wasn't the Slayer, Cordelia was, or Harmony, or Dawn... that rather than Angel being the vampire semi-courting Buffy, it was Darla or Drusilla. That Buffy dies at the end of S2 instead of Kendra. Something that still remains in the same framework of the universe. Likewise a "Harry Potter" fic with no magic, they're attending a school for music, or dance, or... just No. OR a Marvel Comics fic where there are no mutants.
2 - The story is set in the recognizable world, but it is "Alternate Universe" in that a few things were switched - perhaps the Slayer is Willow, not Buffy, or the vampire is Xander, not Angel. Yet somehow, everything except the physical descriptions remains identical - instead of a pretty, confident popular blond ex-cheerleader who complains about the horrible destiny of being the Slayer (canon s1 Buffy) to her red-haired shy friend with awesome computer skills and a rather eccentric and concealing wardrobe(canon s1 Willow), they give us the confident, popular Willow who is miserable because being the Slayer means she can't remain on the cheer-leading squad, and she keeps ruining her good clothing with demon slime, as she often complains to her shy hacker friend Buffy, who is hiding her longtime crush on her best friend Xander... If the whole point of your story is "What would happen if character #4 was given the trials, responsibilities and abilities of #8 then for the love of good writing, keep characters 4 and 8 identifiable as themselves! Otherwise you've failed the point before you began.
3 - Pointless bashing - the sort that goes I the author do not like these things that character 3 did in season 5, so I will write a story set in s2 where I destroy them slowly. And their little dog too.
3b this is not the same as writing a story where in character 3 gets called on something that they did that was rather mean or stupid or just proved to be a disaster. One can point out mistakes without pointlessly bashing. But generally, when you hit ground level, that's enough - you don't need to break out the shovel.
4 - Diverging from canon is a great thing, it can be a fabulous thing (hasn't everyone had something that they didn't like happen? 2 and 8 getting together, 5 dying (and they were your favorite character too!) a romance starting/ending that you disagreed with?) and it can head off later unpleasantries, but... please, please, please remember that if you change this you also need to either remember that b, c, d, and e therefore wouldn't have happened, or give the readers alternate explanations for why all or part of those events still happened. If Reed Richards didn't have an experimental space ship, why would Ben Grimm end up a big guy made out of rocks? If Buffy and Angel never had sex, how did he ditch the soul and become Angelus? If Thomas and Martha Wayne weren't killed in the alley, why would Bruce Wayne become Batman? And ".... because!" is not a valid answer.
5 - Suprise sudden slash. For example, in 5 (okay, maybe 7) seasons, Xander Harris was consistently hetrosexual. He had a couple relationships an more crushes, all of them involving females. So why, oh why does your story open with him and Spike naked and shouting Oh God! Harder, Yes! Or we see things going on as was canon for this season, only... exactly why is Lex Luthor snuggling up to Pete and nibbling on his neck, calling him snookums? If there is a canon same-sex relationship, it's one thing to show them together (please rate accordingly though!), but... if you are using a non-canon relationship, please show us how this started. If the relationship is not only non-canon but a completely different orientation for the character when compared to their canon behavior, then our first reaction as readers is likely to be WTF?!? Some people want warnings for that even if it does build gradually.
5b On a related note, please do not prepare your story with the Magic Mushrooms of Gayness (phrase coined by someone on another site I frequent). You can tell these stories because the characters are - each and every one - in a relationship with someone of the same anatomical gender as themselves. Giles is dating Ethan. Buffy and Faith are doing more than patrolling together. Cordelia and Harmony don't just shake their pom-poms together. Willow and Amy are doing more than casting spells together. Xander and Jesse weren't just friends. Devon and Oz have some really special bonding times after their performances. ... ummm. No. One person being gay - could be canon. One person discovering they might be gay (or bisexual) - plausible. A whole town turning up gay all of a sudden? no.
6 - Everyone Must Have A Partner. No. This is fanfiction, not Noah's Ark. It is acceptable for the story to end without everyone being in a relationship with someone.
7 - The already mentioned 'crossover' where all the author does is substitute this character for that one, yet they still do the same things, fight the same foes, date the same people, and... nothing's really different except their main character's name.
8 - Character Baddo BadGuy has insulted, fought with, and done everything in his power to make Shy-oh-my miserable, yet all of a sudden, he can't live without her, wants to have her as his wife, and... after a lame apology (maybe) and a fangirly description of how hawt Baddo really is, Shy-oh-my is okay with this, and goes off with Baddo to live ever after? WTF? Sometimes I hate you just means... I hate you, and there's nothing redeemable to Baddo. (of whatever fandom you may be familiar with.) Sometimes the jerky guy... is just a jerk. There's no deeper meaning, he's not a good guy deep down - he's just a jerk. Or evil.
9 ... likewise, all the pretty people are not always good guys. Nor is someone automatically the evil fiend because they don't like the main character of the story. It's called giving them depth and dimension. Likewise, to prove just how evil Baddy McVillain is, you don't have to have him going around with a goatee kicking puppies and cheating senior citizens out of their retirement funds.
10 - please don't hold your updates hostage for X number of reviews.
11 - and don't put your author's notes in the middle of the story! Put them at the beginning, or at the end.
Now for the flip side.
Reviews are nice, reviews can make an author's day. But there are a few things that are annoying in reviews...
ths Sux! <- that is not a review. If you do not like a story, tell the author why - is it because the action is wooden? Are misspellings thick on the ground? Are the characters not behaving true to themselves?
You should split up 3/5 and make it 3/7! <- No. I am the author, so I am the one who decides who the characters are paired with (well, me and canon.) Should I ever want reader input about a pairing (and I have never asked for that thus far) I would say so in an authors note (at the top or bottom of the story) and request reader suggestions. Not reader orders.
UPdate soon! <- You mean I should abandon my responsibilities to job/house/children/homework/sleep and update this particular story
right now just because you demanded it? No. Particularly not when that's the entirety of the comment. I like to know if readers enjoyed, and I like to know that they would enjoy more, but I have a life beyond the keyboard.
eeeeeewwwww! How could you write 6/2 together! <- Many authors put predominant pairings in the headers of their stories (some archives demand this, other archives are character or pairing specific). It's often easy to identify - sometimes from the summary - who the main pairing of a story will be. If you see a summary stating that Harry wants to propose to his true love Ginny and it's identified as a Harry/Ginny story, then.... why are you shocked and appalled that the story is a Harry/Ginny fic?
sorry, I think I ranted a little.