Natures
Naive raises Speed 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.
Speed at the cost of Sp. Def — for mixed sets that use both kinds of attack and do not intend to take anything.
Flavor
Likes Sweet, dislikes Bitter. This matters for Poffins, Pokéblocks and Berries: a Pokémon eats a disliked flavor reluctantly, and healing works worse.