I: Any type that wasn’t elemental, psychic, or normal had no usable moves. This resulted in so many cool mons with a movepuddle. eg Pinsir, Scyther, Kingler
II: Johto was designed to be half the game (gym difficulty, pokemon availability) because Kanto existed, but Kanto was ded
III: slow frail mixed attackers
IV: tie between D/P surf speed and “saving a lot of data...”
or maybe it was that HG/SS were so good that they could never live up to that standard again
V: Encounter rate. BW is unplayable most of the game until you get repels and then after you beat the E4 it’s unplayable again without remedy after the wild levels get jacked up to 55-75.
VI: tie between new levels of powercreep and handholding
VII: more powercreep
VIII: Gamefreak
