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>gen 2
Breeding mechanics, hold items, pokegear-type devices, lots of other shit
>gen 3
Abilities, natures, double battles as the main competitive format, IVs/EVs
>gen 4
Physical/Special split, online battling and trading
>gen 5
????????????
Uhh... infinite TMs? Oh wait, that's gone....
>gen 6+
3D, battle gimmicks