Impactful changes per generation (updated)
Gen 2: 
-Special Atk/Def split
-Critical hit rate lowered
-Steel type
-Held items 
-Leftovers
-Spikes
-Sleep Talk+Rest
-Substitute blocking status and other effects
-Hidden Power
-Baton Pass
-Wrap/Fire Spin nerf
-Hyper Beam nerf
Gen 3: 
-Natures
-510 EV limit, before Gen 3 all stats could be maximized 
-Abilities
-Choice Band
-Sleep Talk+Rest nerfed
-Toxic doesn't become regular poison upon switch
Gen 4: 
-Physical/special split
-Stealth Rock
-Choice Scarf
-U-turn
-Draco Meteor/Outrage
-Life Orb
Gen 5: 
-Explosion/Selfdestruct nerf
-Scald
Gen 6: 
-Fairy type
-Steel nerf
-Mega evolution
-Grass-type are immune to Spore/Sleep Powder/Stun Spore
-Electrics can't be paralyzed
-Weather abilities nerf
-Knock Off
-Critical hits weakened
Feel free to add
