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Why did Gen 2 had so much effort put into it? Why are newer games so mediocre in comparison? When compared to gen 1, it added:
>Day/night cycle, nocturnal Pokemon
>Massive graphic upgrades: better sprites, colors, town design
>More fleshed out music, SOVLful melodies that are memorable for a lifetime and make your heart clench
>A toned down story that focuses less on being a hero against some comical supervillian, and more on local folkore and its legendary Pokemon
>Incredibly comfy atmosphere that hasn't been matched yet
>More memorable and meaningful interactions with NPCS (helping Kurt, Whitney having a fit and not giving you your badge, etc)
>Improved inventory
>Phone, NPC and gym leader rematches
>More unique types for Gym leaders (flying, bug, ghost, ...)
>Introduces 2 new types, steel and dark
>Has you battle Red, your previous character in Red/Blue/Yellow
>Has you visit Kanto from a new perspective, after your previous journey, you can see how it has evolved, even if it's a brief visit
>You can customize your room
>Based and unique Pkmn with special type combinations. See Lanturn, Scizor, Houndoom, Heracross, etc.
Gen 3, 4, and 5 have their own merits, but they didn't try as hard to be good. Gen 6 and after is just decay and not even worth mentioning.