Fan fic cliches are storylines and themes that are oft used, and although this doesn't mean they don't produce great stories, they occur commonly not only within Supernatural fandom, but across many fandoms.
In January 2010, missyjack created a "fanfic cliche bingo" card, to illustrate how many fanfic tropes had been Kripked.
The tropes are discussed in detail here
They show covers many kinks from Fanfic, most famously Hurt/Comfort as noted by acafan Henry Jenkins: "Heck, this series is one long Hurt/Comfort story." (Source).
At the end of Season 6, all of the fanfic tropes on the card below had been featured on the show except Mpreg.
In addition there's been Wingfic, Crossdressing, Dean becomes a vampire, Kid Fic, and Outsider point of view. A small memorial has been built where the fourth wall used to stand.