Natures
Rash raises Sp. Atk by 10% and lowers Sp. Def 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.
Raises Sp. Atk, lowers Sp. Def. Usually worse than Modest, because it weakens the side special attackers get hit on most often.
Flavor
Likes Dry, dislikes Bitter. This matters for Poffins, Pokéblocks and Berries: a Pokémon eats a disliked flavor reluctantly, and healing works worse.