Plot: I've heard it has the "best plot" of a Pokemon game perhaps, with some exciting moments and such, yet at the same time this also means more linearity and pushing the player along with cutscenes. The main game is better than XY at the very least. Decent amount of classic cameos, but nothing B2W2-level.
Difficulty: I've heard the game isn't super challenging, the opponents still use less Pokemon than before, yet with the level scale system back from BW it's harder to overlevel compared to XY. At the same time the Totem Battles can be tricky with some interesting combos, the opponents now actively target weaknesses and switch, and many of the New Pokemon are on the slower side so using them means the opponent will get in an attack at least instead of you just sweeping through them.
Pokemon: Not as varied as XY, but a decent amount of classics---the New Pokemon seem to have a "Johto syndrome" where they're a bit pushed to the side in many cases with low encounter rates and such. Also they're all pretty slow, including ones who you think would be fast. Some classic Pokemon got buffs (one got a nerf), including stat upgrades and new Abilities. There's not many non-Alola Dex Pokemon available in the post-game right now, so you'll need to rely on PokeBank if you're going for 100% National Dex. There's no in-game National Dex, presumably being saved for Bank.
Music: Varied and really nice, weakness music is possibly the default battle themes but everything else is good---"best OST since Black and White".
Graphics: Frame rate still takes a dip pretty regularly, and the addition of Trainers lead presumably to the removal of Triple and Rotation as well as Hordes. Otherwise the region looks nice.
Post-game: Neat new system that makes competitive training pretty interesting and fast, though requires some set-up. Multiple Legendary Pokemon to catch and a Battle Facility that's fairly challenging. About XY-level from what I've heard. Downside is very few non-Alola Dex Pokemon to catch and not a lot of new areas that open up. Do get to rematch various major Trainers though such as Trial Captains, and use more than just 2 Pokemon like the Battle Chateau rematches in XY.
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