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Too much story and forced interactions.
Took away supplementary key items.
Basic features and mechanics that were in the main game of in previous gens are moved to to the post-game in the Gen 5 games.
Has a pokedex filled with questionably designed pokemon that also take way too long to evolve and majority of them have bad movesets that don't support their stats.
Cumbersome cross-gen transfer system.
Introduced new battle formats like Triple/Rotation Battles that ended up being barley utilized, either because the weren't really fun or because they slowed down the framerate.
Started a unrecoverable powercreep.