Natures. I hate 'em. I love 'em at the same time. I didn't bother with natures until Diamond and Pearl were released.
Now I soft-reset at the start of each game so my starter has the right nature; I can't have a Pokémon on my team unless I catch it with the desired nature or I waste hours breeding the Pokémon for the perfect nature on another game; soft-resetting to before the egg hatches, then trading the egg over and hatching it on the game it's meant for.
I know in the story mode, natures make little difference; but I can't help myself. I think I'm OCD about it. When I sit down to play the game now I get quickly put off because I spend ages breeding for the right nature rather than just enjoying it!
Anyone else suffering like me? :P