1. People who don’t like each other in canon and are having sex and/or are in love in fanfic. It’s a staple of slash, and of heterosexual romantic comedy, for that matter. From It Happened One Night in the movies to the TV show Cheers you see a bickering pair, whose differences of class or temperament or interests are apparent and whose antipathy is generally expressed humorously. We the readers or viewers are supposed to recognize that there’s an underlying attraction. It works for slash, particularly if there’s a kind of grudging admiration between the two in canon in spite of their differences. As ringthebells says, what makes two characters slashable is often emotional intensity. Sometimes that intensity shows up in the form of hating each other’s guts :-). As the words to a filk I love declare (“Snape and Harry” to the tune of “Love and Marriage”):
Beneath that mutual loathing
They want to rip off each other’s clothing.
If the author is skillful s/he’ll get us to the point where they are ripping off each other’s clothing and it will feel realistic.
2. Related, but not the same, as the first: opposites attract. Best friends or lovers whose canonical hallmark characteristics are in opposition to one another, but they’re crazy about each other anyway, or maybe even because of the difference. Iceman and Pyro are a perfect example.
3. Laughing in the face of danger. Humorous banter during fight scenes or when in peril. It’s a comic book staple, an action movie necessity and a fanfic cliché. But I can’t get enough of it, if it’s done well.
4. Superpowers as liabilities. One of the things I love about the X-Men is how ambiguous their powers often are. It’s certainly wonderful to have special gifts others don’t have, but many of the gifts have downsides, as well. One of the things that fascinates me about Cyclops is his inability to control his gift (and his attempts to control everything else at least partly as a way of making up for that). Similarly, I love stories about telepaths who “hear” things they don’t want to know, Rogue struggling with the voices in her head or her inability to touch, mutants who for reasons of love or fear or just desire to belong want to be “cured.” In Star Wars I like the stories where the downsides of Jedi life are seen, too. I’d love to see some HP fic where being a wizard is presented as not all it’s cracked up to be – with Harry or Hermione or some of the other Muggle-raised kids at Hogwarts sometimes wishing they were regular kids in a regular school.
5. Return of the Bad Guy. You know, the stories where the hero thinks the villain is dead, in jail, retired, or otherwise out of commission. Little clues suggest the hero is mistaken about this. Tension mounts and then suddenly he’s there and attacking, taking all the characters in the story (but none of the readers of it) by surprise. There’s a gloriously exciting battle scene as the hero has to get his bearings, adjust to the impossible (“It can’t be him!”) and manage to vanquish the villain. In general, there are plot holes in these kinds of stories that you could drive a truck through, and I just don’t care. Make it exciting enough and I will forgive almost anything.
This was another fun one! I don't know who comes up with these, but thank you whoever you are.