Within Torchwood fandom, a truly mind-boggling number of Jack/Ianto Hurt Comfort Fics involve the characters showering together—often not even romantically, but simply because one or the other is hurt and needs help, or is untrustworthy and has to be watched.
It seems as if there is no one who hasn't written the "Countrycide" follow up fic.
Lots of Fix Fics were written after Children of Earth, either changing the end so Ianto (and occasionally Steven) doesn't die, or bringing him/them back to life somehow. Time Travel is sometimes involved.
An abnormally high number of Torchwood/Anything Else fics involve a Jack/Ianto/Other Universe character. Especially if the other universe is Buffy the Vampire Slayer.
Torchwood crossovers with the Buffyverse actually have their own fandom specific plots.
Buffy either joins Torchwood and has a falling out with the Scoobies or has a falling out with the Scoobies and then joins Torchwood.
Everyone from the Buffyverse practically worships Ianto's coffee.
If it's a Buffy joins Torchwood fic, she and Jack butting heads over stuff, mainly her tendency to want to be in charge.
Spike will always get mistaken for John, and vice versa, often leading to someone getting attacked.
Dawn ships Jack/Ianto.
Torchwood calls bullshit on demons, the Scoobies on aliens.