I am just going to start off with the things that annoyed me about the game:
The first was the Disney Worlds. My main issue was how long they were. In previous Kingdom Hearts games, you had plenty of story in-between and the levels were far shorter. You also revisted them in KH2 which helped alleviate some of the repetitive nature that some of them had. There were also some that just felt... not as good. The Pirates of the Caribbean world felt like it was incredibly short, story wise - but it gave you a lot to explore. The Monsters Inc. world felt like it was room after room of combat challenges, meanwhile the Big Hero 6 world was just a single large map and a garage with Pooh's just being 3 of a very similar minigame on a very tiny map. There was also the issue of some of the worlds feeling ultimately inconsequential, as if Sora and the gang amounted to nothing. In particular, the Frozen and Pirates worlds felt like this.
If I were to rank the worlds I would say that:
I love: Twilight Town, Scala ad Caelum, The Final World
I like: Olympus, Toy Box, Kingdom of Corona, and The Keyblade Graveyard
I am more Neutral about: Monstropolis.
I dislike: San Fransokyo, Arendelle, and Caribbean
Also I just wish there were *more* worlds to explore. I was hoping we would explore Radiant Garden or Traverse Town again. Hell, I am more surprised we didn't see Daybreak Town in any shape or form.
The story, while being mostly perfect and ties up a lot of loose ends - it leaves a number of questions open that annoy me to a great degree. Cool, we finally know that Braig is Luxu, but did he lie about his mission given to him by the Master of Masters? Why would he openly interact with people when the Master of Masters told him to simply watch and protect the box? Speaking of which, who is the Master of Masters? After Olympus I was thinking it would be Braig, but that was dashed. What happened to Ava and why are all the other Foretellers still alive? How did they survive? Why was Ephemer helping Sora? Was that what Ava meant when he left to help the lead the Dandelions? Did Kairi make her way back to the others without the need for Sora's help? What was all that about Sora and young Xehanort suggesting he can't be saved? Was that in reference to the secret ending? Also, who is the "girl" that Axel and Siax keep referencing? Gaaaah, damn you Nomura. You finished the Dark Seeker saga, like you said you would, but now we gotta wait for Kingdom Hearts 4 to get closure or answers to any of these questions.
Another thing I didn't care for was how easy the gameplay was. I died a single time during my proud mode playthrough and that was simply because I wasn't paying much attention to the fight between Aqua and Vanitas. The lack of a Critical Mode also is a bit disappointing.
Now, for the things I love:
It has some of the worlds that made me genuinely just stare at the screen for nearly an hour, listening to the music or just staring at the beauty of it. Scala Ad Caelum and the Final World are just beautiful and tranquil with Twilight Town always being one of my favorite worlds in all of Kingdom Hearts.
The story, disregarding what I stated above - pretty much tied up most of the loose ends and made me tear up multiple times. Seeing the return of Aqua, Ventus, Terra, the return of Xion and Roxas, Namine finally being able to have her own life. All the best girls are finally back and able to be happy and that makes me happy.
The core gameplay is so much fun as well. The switching of forms for the Keyblade was excellently done, the attractions were quick and easy to use (by about the third world, I just used it and finished it immediately for some punishing damage and it worked very well), plus it was nice to be able to have a good list of moves again that I could customize.
Graphically and musically, this is beyond the best game of the franchise and the new main menu theme is breathtaking. I love it so much.