Pee-oh-keh-e-em-oh-enn - Pokemon!
This game has almost everything I want in a Pokemon game. I even feel a bit bad saying "almost" but I felt like a lot of the Unova region Pokemon were kinda lame, and found myself using a lot of repels. That's just me, though, and besides, through trading (or using PKHeX for you weasels out there) you can get nice vintage Pokemon into the game.
But hey, let's get more positive. The story is interesting, and I want to see more stories like this in Pokemon. Multiple times, the game acknowledges that years have passed since the events of B/W, and things changed. Team Plasma has split up, but there are still people causing trouble. The protagonist from B/W has disappeared, and you are just trying to get those damn gym badges to challenge the Elite Four, and a new champion, Iris.
Throughout your quest, you'll notice that the soundtrack is really, really good. That might be a redundant description, but this really might be some of the best soundtracks of the series. Most of the soundtrack is new, so they were not being lazy in the sound department.
Graphically, the presentations for gyms are epic, and looking at battles makes me wish Pokemon games went back to using sprites. I don't agree with a few decisions, like the way Castelia City makes the game zoom out, leaving sprites looking like a shrunk pixelated mess. Black City also suffers from this, but these are small complaints.
Let's get to the gameplay, holy hell. On a personal level, I love that there's a lot of post-game content. The World Tournament, a lot of special battles, Black City being unlocked and having an upgraded Black Tower. And even before then, you have pokemon breeders that you can re-battle by leaving the area they're in and coming back to them. The previously mentioned World Tournament even brings in Gym Leaders from generations. Nimbasa City still has the big stadium and small court to help you raise your Pokemon through battles, which is a nice touch to have that, in case you don't feel like seeking out rematches in different routes.
You also get more than just a certificate for seeing all the Pokemon in the Unova Pokedex. You'll get access to a new area with a shiny Haxorus and other Pokemon not usually found in Unova. There's more goodies for you if you catch all the Pokemon in the Unova Pokedex and the national dex (Oval Charm, Shiny Charm).
This all may seem like small details, but it adds so much to the experience of playing the game after beating the Elite Four for the first time. Compare all of this to the poor post-game of Pokemon X/Y. And the features I mentioned were just the ones I found to be important, there's a bunch more, like the Join Avenue and the items you can get from there, hidden grottos, new move tutors, etc. There's even little details like asking you if you want to use another repel if the one you just used runs out, or moving items in a party instead of having to put them in a bag, leaving the party screen, then going back in to give the item to another Pokemon.
It's everything a sequel needed to be, and I loved it for all the improvements it has over B/W. And seriously, that soundtrack, man. If you haven't played this, at least give the soundtrack a chance, it's really good.