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Gen 2:
>Has a day/night cycle
>Has the previous region as post game
>Has apricorns + ball making
>Has time-based events outside of berries
>Has all Pokémon available at the time obtainable in the game
>Has day/night battle music
>Has good worldbuilding and storytelling that builds on the lore established in the previous entry
>Has fun side-content
Gen 3:
>Removed day/night cycle, now just permanent daylight
>Removed two regions, completely isolated from previous regions
>Removed apricorns and ball making, hope you enjoy your daily Oran Berries
>Removed time-based events like the weekday items and rival battle on Wednesdays
>First Dexit in the franchise, removed half of the Kanto Dex and almost all of the Johto Dex
>Removed day/night battle music
>Lore is completely disconnected from Gens 1 and 2
>Removed Bug-Catching Contest and Trainer House, enjoy your Contests which give you nothing worthwhile
Gen 3 killed Pokémania.