I do not write. I read. Sometimes I criticize. Mostly I read Lord of the Rings or Harry Potter based fan fiction.
Gripes:Mary Sues. Shameless self-insertions that are really just a veiled insult to the reader. Fine if you like fantasizing about bonking Orlando Bloom in the privacy of your own mind, but to subject me to your poorly disguised OC getting it on with Legolas is just beyond cruel. Just bloody stop it already. Slash/Yaoi. The poor man's, or rather poor woman's, Mary Sue. The INVARIABLY female author cravenly takes over a canon male, takes the extra appendage in stride and goes off to bonk Orlando Bloom, Tom Felton, etc. etc. etc.
And the predictable accusations of homophobia when caught.
It is just so insipid and so mind-numbingly boring. Orthographic ignorance, grammatical atrocities and the casual molestation of canonical names. You know who you are. Confusing your (quasi) homophones. For example conscience versus conscious. If you're incapable of keeping above errors out of your brief synopsis, you don't deserve consideration. I'll assume by default that your writing sucks. "Don't like, don't read". The hallmark of a bad, vain and defensive author trying to wave off any form of criticism. Narcissism much?
And seriously, how the hell I am supposed to find out if I don't like your story, if I do not at least read part of the first chapter? Yes, I filter on search and I always read the summary, but sometimes crap just slips through.
Some other gripes:When the story serves the Romance and not the other way around. The silly shipping wars in Harry Potter are usually a good example of this. I am definitely not interested in a 70K story about the insipid courtship of two characters. Boring. Nothing of interest ever happens. Twins. I know twins in real life. They do not behave at all like the Weasley twins. Twins are not automatically mischievous, nor are they natural pranksters, or whatever insipidity is driving your perception of Elrond and Elros, Elladan and Elrohir, etc. etc. etc. The word 'Unsung'. Whenever I encounter that in the synopsis, I just roll my eyes and prepare for a descent into the pits of lame. If you're going to write a shameless self-insertion, have the grace to disguise it better. This also goes for suddenly appearing brothers and sisters to canonical characters. You (and your friends) joining the Fellowship, taking Orlando Bloom (-- Well, Legolas, but you know what I mean --) to the mall (WTF?!), going to Hogwarts, etc. etc. etc. Still Mary Sue and still very boring for anyone that is not you. Drabbles. Don't get me started. A collection of well-written drabbles is the obvious exception. Songfics. Finish what you start. Take your time; it's not like we're paying you. But, sheez. I've seen way too many good fics abandoned because the author got distracted by new ideas, new fics. Which will then also not be finished. Repeat.
If you really want to abandon a story, put your readers out of their misery and tell them.
Silmarillion and Lord of the Rings gripes in no particular order of importance:One of the Valar addressing one of their charges as 'little one' or 'child'. Names exist for a reason. Use them. Nobody likes condescending, patronizing deities. Elves do NOT have pointy ears. At most, and even that is disputed, Tolkien described their ears as 'leaf-shaped', not as resembling Star Trek Vulcans. It's their beauty and their eyes that give them away. Sauron is evil. He is not misunderstood, tragic or cute. If you want to redeem him, write it well, but try for a little more than that Melkor was just big meanie to him and he had no choice. Bad and tragic elves do not need the love of a good woman to change. It didn't work for Eol, it won't work for the sons of Feanor. If you put someone from the present into Middle-Earth, be prepared for the language barrier or have a pretty good hand-wave to explain it away. Movieverse. Just don't. Elrond does not have an unsung, unknown second daughter. I'm talking to you, Mary Sue. Aragorn will not fall for your OC. His romance with Arwen is the leitmotif for his actions in Lord of the Rings. Your Mary Sue will not distract him. Legolas romances, and I don't care with whom, get filtered out. I hate you, Mary Sue.
Harry Potter gripes in no particular order of importance:Harry gets a really 'cool' name like Harrison, Hadrian, Harold or some other assorted nonsensical wish fulfillment. If the name Harry is good enough for the House of Windsor, it is good enough for Mister Potter. The Most Ancient and Most Noble House of Deus Ex Machina. From a casual mention of Black snobbery in OotP this has grown into a fanon cliché of monstrous proportions. In all of the books, there is NO evidence of wizarding nobility, and even if such a thing existed it would not grant instantaneous power-ups for the protagonist. And it makes Harry part of the same reprehensible system as his side claims to fight. Some authors even manage to use it to turn Harry into an entitled little wanker who lords it over whomever does not have his exalted ancestry. As a follow up, don't even think about giving him a signet ring to wear. Nevermind he is Harry Potter; he has to be extra special, exalted and elevated above his classmates. Extra bonus points if he is entitled to and demands deference from people he does not like. The heir of the Most Ancient and Most Noble House of Douche has come to Hogwarts. Line Theft. Oh noes! Not the Line Theft. Hand-waving, cliched insipidity. So Mote It Be. Just stop it already. Whether used in conjunction with the House of Deus Ex Machina or to just illustrate the validity of a sworn statement, this has become a terrible cliche, indicative of a plot full of hand-waves. There are countless other methods of keeping an oath or verifying truthfulness, each with their own limitations, such as an Unbreakable Vow, Verita Serum or a magical contract. A good Canon example of the latter would be the DA's membership list as it was written by Hermione. The Wizengamot. The Wizengamot is wizarding Britain's high court of law. It is not a hereditary parliament or House of Lords, where members of the Most Noble and Most Ancient Houses of Deus Ex Machina drink tea, oppress the masses and say "So Mote It Be" a lot. Just don't. The Goblins. They're bankers. That should tell you all about them that you need to know. They do not like wizards. They're in it for the money. They're not Deus Ex Machina Elves that can put everything to right for the protagonist. They also do not do inheritance tests, resulting in the protagonist being the heir of several Most Ancient and Most Nobles Houses of Deus Ex Machina. Nor do they do Magical Ability tests, where the protagonist gets this magical printout of all his abilities in percentages. It is boring. Harry is not mega rich. He will not be informed by Goblins of his seven vaults, filled with coins and indexed to the Knut. He will not receive magical printouts listing magical and Muggle investments to rival the holdings of Warren Buffett. Nor will he receive a list of magical properties he owns around the world. There might be some cash stashed away for him to find, but by Merlin's soggy underpants, keep it realistic. Dumbledore. Good, misguided or bad, however you choose to write the man, he does possess a formidable intellect. If you write him as an evil force, make him genuinely dangerous. He will not use 'For the Greater Good' in every other sentence or thought. He will have a plan. And backup plans. He will be adaptable. A good Dumbledore may listen to reason and should be able to be persuaded to change his plans. Therein might lie a challenge for the protagonist. Movieverse. Just don't. If you have to get Harry a snake for a familiar, please don't make the damn serpent his personal encyclopedia for all things related to the magic world. Unless you're a lazy writer and don't want the protagonist finding stuff out through trial and error that is. In that case, go right ahead and give him his Deus Ex Machina snake. Flat, exaggerated characters. Ron Weasley as just a greedy, jealous slob with poor table manners. There's so much more to the boy than just that. Ginny Weasley as the Potter obsessed fan-girl. Molly Weasley. Severus Snape. Albus Dumbledore. Frigging Voldemort. Give them some depth already. The shopping spree. Have you any idea just how boring that cliché is? Especially when Harry buys one of those mega cool trunks. Yawn. Straight up rehashing of canon or retelling of canon events through page upon page of dialogue. We've read the books. We know all this stuff already. As a follow up to my previous point, if you have to do a Sorting Hat song, either write your own, or just casually refer to the canon song; don't cite canon verbatim. Gred and Forge. So original, like no other fanfiction author has done that one before. And don't get me started on the inevitable 'twinspeak' where "one twin..." "finishes..." "the other's..." "sentences". Cliché, boring and quite frankly annoying. Referring to Sirius as Harry's 'Dogfather'. Sirius calling Harry 'pup'. Remus calling Harry 'cub'. It wasn't funny or endearing the first time, and it doesn't get any better the more it is used. What if Harry was... diabetic... blind... burnt all over... photo-phobic... bipolar... retarded... Yeah, that will really draw your readers in. (There are actually non-crack fics out there based on the premise of Harry being retarded. What the fuck is wrong with some people?) Slapping 'maxima' on a spell. Seriously? I've read fics that used "crucio maxima'. As if 'crucio' wasn't enough. Protego maxima. Reducto maxima. Rubbish. Just invent something of your own, instead of this maxima tripe. It's lazy and it reeks of movieverse. Using 'Dumbledork' and 'Moldyshorts'. Just don't. Harry becomes an Animagus. And he's always a mega-cool, preferably magical, animal. I have seen Shadow Panthers, Phoenixes and even bloody dragons. It's trite. It's cliché. It's boring. Pranking. It's just a douchie term for bullying. The obsessive, almost pathological, need to endow characters with a middle name. Marriage contracts, betrothal contracts and harems. The first two, don't. The latter, unless you're writing humor/crack, please see a therapist. Referring to spells by their incantation. It's the killing curse, not the Avada Kedavra. People are not 'Avada Kedavrad' or 'AKed', they are killed and/or murdered. Telegraphing what is going to happen and then writing chapter upon chapter of drudgery to take your reader there. True love, soul bonds, soul marriages, etc. If shipping is so important to you, just ship, but if the ship needs "True Love" to sail, it could never have sailed without. It's up to you, the author, to grow their relationship and since it involves teenagers, it will be awkward. "True Love" is just lazy. Disclaimers: Mostly pro forma anyway, but if you have to have one, please, for the love of Morgana's sweet knickers, keep Warner Bros and publishing houses out of it. J.K. Rowling is the author. Don't be a grovelling corporate flunky.
That all said, if you have to break a rule, break it good and hard, and break it with style. One-shots using some of these tropes can actually be pretty funny.