Here is what I know.
Gen I: Pokemon Battling,evolution.
Gen II: The Special Defense and Special Attack Stat,experience bar shown in battle,pokemon eggs,evolution through happiness ,gender(thanks to relic castle king),breeding,day care.
Gen III: Abilities,Double battles.
Gen IV: The Physical/Special Split. Evolution through certain moves,pokemon following you.
Gen V: Hidden Abilities, triple battle,rotation battle,shaking grass,rippling water,dust spots,TMs can be used multiple times instead of once,moving Pokemon battle sprite.
Gen VI: Mega Evolution,sky battle,horde battle,inverse battle,soaring,wonder trade,supper training,pokemon amie,O power,dex nav,news nav,ridding on Pokemon.