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It's very different from all the other pokemon games. There's no leveling up your pokemon, instead you have a percentage that shows how close they are to you. Raise the percentage enough, and they evolve when all requirements are met.
Another big thing. All pokemon know only one move, rather than four. They will gain a new move upon evolving. Raising the aforementioned percentage will sometimes raise the moves attack power.
Despite these changes, it's definitely worth it. I loved playing this game.