Natures
Sassy raises Sp. Def by 10% and lowers Speed by 10%. It is applied at the very end of the formula, after IVs and EVs, so the higher the stat, the more the nature gives and takes.
Sp. Def for slow Pokémon. Under Trick Room and with Gyro Ball the lower Speed stops being a drawback.
Flavor
Likes Bitter, dislikes Sweet. This matters for Poffins, Pokéblocks and Berries: a Pokémon eats a disliked flavor reluctantly, and healing works worse.