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The story is completely different, many new towns and dungeons/caves, 3 new gym leaders, all of the returning gyms have different puzzles (and specific music), 300 pokemon in the regional dex (you can catch a riolu before a single badge) and different victory road.
Not to mention the best post-game content: pokemon world tournament where you can fight all the gym leaders and champions from all regions, white tree/black city are now places you can battle in based on floors (kinda like the battle pyramid but not really) and... just look at BW2's map compared to the firsto one
