1st and 2nd Generations: Classic Generations, the central themes are the creatures themselves, the collectors aspect, the pokémon fights against gym leaders and the league. The evil team is just pokémon Mafia. The setting is plain and simple.

3rd and 4th Generations: Lore-Based generations, the central theme is the mythology of the pokémon world and his gods.

5th and 6th Generations: Plot-Based Generations, the central themes are the stories of the evil teams and their ideologies.

7th and 8th Generations: Character-based Generations, the central theme is the interaction among psychologically tridimensional characters, and sometimes their psychological development and analysis (like Lillie).

More in general, I think the thing can be divided in:

1st-2nd-3rd: Simple Generations, the focus is on the pokémon, plot, characters and settings are simple.

4th generation: intermediate Generation, it has intermediate aspects between simple and complex ones.

5th-6th-7th-8th Generations: complex generations, the focus is on the plot, the characters and the settings.

Do you agree? If not, why? Do you propose another classification?