An incomplete list of things I'd be perfectly satisfied never to see again, thank you, in Sherlock fanfiction. In no particular order:
1. Mystrade seriously presented as canonical. There is no evidence that Lestrade and Mycroft have ever even met.
2. Hobbit crossovers/fusions. Or Star Trek crossovers/fusions. Or any “ooh these two completely different characters in separate universes are played by the same actor!” crossover/fusion/meetup.
3. Vampire stories. Werewolf stories. Sherlock-Goes-To-Hogwarts stories.
4. Any crossover involving more than one fandom other than Sherlock. Honestly, what the hell makes you think any sane person wants to read a story crossing Sherlock over with the Star Trek reboot and Doctor Who and Supernatural and the entire Marvel movieverse and James Bond and Criminal Minds and some anime 'verse I've never even heard of?
5. John's fucking jam. The entire “John's jam” trope. For some reason a lot of the fans are inanely amused by the idea that John has a jam fetish. This is borrowed from a “Hark, A Vagrant” comic that featured ACD's Holmes and Watson – in which the character who liked jam wasn't even Watson, but the annoying comic-relief character “New Watson”, later known as Stupid Watson. Before you write any story or even Facebook posting involving John and jam, remind yourself of three things: a) there is only one mention of jam in the entire nine aired hours of the series, b) it is metaphorical, and c) it is in the context of someone getting hit by a train.
6. Any story or kinkmeme prompt involving the alleged “real reason” Donovan doesn't like Sherlock – because he stole a case from her, because he wouldn't let her steal a case from him, or (all too painfully often) because they dated and he dumped her or she once hit on him and he shut her down, or basically anything other than because she's a good cop and really thinks he's a danger to the public!
7. Any use of the plot, or plot element, or subplot element, or motif, or even faint mention of Sherlock's supposed, and supposedly Deeply Significant, inability to say “I love you”.
9. Elements from the Guy Ritchie films, including Gladstone. This goes double for any story in which BBC!John is portrayed as not merely a gambler but a compulsive gambler.
10. I will automatically hit the Back button at any use of the nicknames “Sherly”, “Mykie”, “My”, or “Lock”.
11. High-school fics or Teen!Lock.
12. Eating-disorder fics, particularly Anorexic!Sherlock.
13. John portrayed as utterly falling apart after Sherlock's death, paralyzed or immobilized by grief, especially in fics set months or even years after the Fall.
13a. John portrayed as utterly alone and friendless after the Fall. Even if John's friendship with Lestrade doesn't survive TRF – hello, Mike Stamford? Bill Murray? Harry? Ella? Remember, it was Sherlock, not John, who said “I don't have friends”.
13b. John portrayed as utterly alone and friendless after the Fall because Harry and Mike and Bill are all dead. Come now, let us not get gratuitous.
15. Ella-bashing. If John's been misdiagnosed, which is at best arguable (I seriously question Mycroft Holmes's qualifications as a psychodiagnostician), it's in large part because he hasn't been fully honest with his therapist.
16. Any use of the phrase “A certain _____” in story synopses: referring to Sherlock as “a certain consulting detective”, or pairing Mycroft off with “a certain Detective Inspector”, etc. etc. and thuddingly unoriginal etc.
17. MorMor, particularly if it involves the nickname “Tiger”. Get back to me when we've actually met Sebastian Moran. Fannish speculation on a single shot in “The Hounds of Baskerville” does not count.
A few tips for writers:
John is blond, not blonde. Blond is masculine, blonde is feminine.
Names are invisible. Use them as many times as you need to: don't (particularly in the middle of a sex scene) suddenly jump to calling Sherlock “the detective” or John “the blond”.
Singular, Holmes: Sherlock Holmes.
Singular possessive, Holmes's: Sherlock Holmes's brother Mycroft.
Plural, Holmeses: the Holmeses glared at each other.
Plural possessive, Holmeses': John listened to the Holmeses' incessant bickering.
If you ever use the form “Holme's” I will mock you relentlessly and you will deserve it.
In speech it is “Baker Street”, not “Baker St”. No one can pronounce an abbreviation. If you ever refer to it as “Bakers Street”, see above under “Holme's”.