Oh dear...Where do I start.
1) Sora lands in Land of Dragons, he's searching for the Organization and Riku, but he interrupts that go to on a mini-adventure here.
Actually, the fact that the Disney worlds were nothing but filler was bad enough.
Maleficent being a joke of a villain in this game.
2) Sora, in his genius, decides it's in his best interest to do a sing-a-long before continuing the search for his friend who may be in danger.
3) A huge ton of heartless assaulting TWTNW with Maleficent and Pete about to attack it, if you go back to the screen, ALL of that has vanished.
4) Riku taking off his Dark Coat to reveal clothes that could NOT have been on him while he had the Coat on.
5) The PoH becoming useless.
6) the fighting going from semi-realistic to borderline DragonballZ (See, a combo in KH1 with all Combo+ attatched to it, using ragnarok and all manner of things, compared to mashing X in Final Form).
7) The awesome Dark City that I wanted to go through since I saw the secret movie of KH1 being only 4 screens, two of which were just transitional screens the size of an alley.
8) The Org uses CoD's to travel around, but somehow Riku finds a two-seat hovercraft before fighting Xemas's Dragon Nobody.
9) DiZ disappearing until the very end.
10) Yuna, Rikku, and Paine are put into the game as fairies...
I could go on and on, I'll just edit in more later.