Here's my dorky revelation of the day:
I can't stand having a Pokémon party where the Pokémon are at different levels. They have to all be at the same level, or at least within 2-3 levels of each other. Also, I can't have more than one of the same Pokémon in my party, and it has to be properly balanced between types to minimize any weakness. I even go so far as to level up my friends' Pokémon if their party is really unbalanced (starter Pokémon or a tank at a really high level, all other Pokémon at low levels).
Me engineer. Must optimize ALL!