I think it goes:
Shipping, which is romance between two characters. (Example Rainbow Dash and... just about anybody)
Dark: Speaks for it self. Pretty much anything with death.
Sad: Speaks for it self. (Example My Little Dashie)
GrimDark: Anything with murder,death,scary monsters, or cupcakes.
Comedy: Yeah yeah yeah speaks for it self. I'm even writing one.
Adventure: Speaks for it self, but I think it usually pairs up with Dark, and Crossover.
Crossover: Crossing MLP over with X. (Example Rainbow Dash and gang, become Spartans from Halo)
I'm just going by what I see on EQD though. I don't really read a lot of FanFic, even know I'm writing one
!