Let me recap:
Gen I: started it all, had some glitches, but pretty much core. Gamplay revolved more on collecting.
Gen II: reinforced Gen I's combat mechanics, introduced new communication methods, introduced concept of version-exclusive Legendary 'mon (Ho-oh and Lugia), introduced baby Pokemon, introduced Day/Night system
Gen III: revamps Berry system, reinforces exclusives, added some new baby Pokemon of older gen families (Azurill, Wynaut), FULL COLOR, reinforces Forme system (Castform, Deoxys), implementation of remakes in every Gen (except V)
Gen IV: Shifting to the DS: Emphasis on new Evos for older gen Pokemon (Magmortar, Electivire, etc.) first 3D games (map rendering only though), expands thoroughly on lore (Sinnoh dragons, Regitrio sidequest for Regigigas, Arceus plates and Sinjoh Ruins)
Gen V: DSi-based games: Mix of suspiciously similar previous gen elements storywise (namely I and II) and more Pokemon replacements for competitive battling (eg. Magnemite for Klink), balanced, nerfed and buffed moves (I'm looking at you Dragon Gem) and reinforcement of communication based features (Entralink, Dream World), focus on fanbase hate due to the notorious Vanillite, NO NEW EEVEE EVOLUTIONS OR ANY EVOLUTIONS WHATSOEVER FOR PREVIOUS GENS
Gen VI: 3DS: PSS and further Internet-based activities, Horde Encounters (taking cues from Dex entries alà Seviper and Zangoose, or Durant and Heatmor), overhaul of breeding (Destiny Knot), devamping of Global Link (PokéMileage Club), New Eevee evo, etc.