The first game of the series was really the only one I loved unconditionally. It was fresh, colorful, and more than a little strange, but not in a bad way. More importantly, the story was clear and made sense. Well, as much sense as a story about shadow monsters devouring planets can make, at any rate.
Once KHII dropped, the series started getting bogged down in inane, metaphysical terminology and absurd plot twists that really served no purpose whatsoever. I played KHII start to finish out of loyalty to the original game, but by the end of it, I realized the series would never be the same.
I also wasn't fond of the direction the combat took in KHII. It seemed to pull inspiration from Final Fantasy VII: Advent Children, by which I mean almost every aspect of it was incredibly flashy. Lots of flying around, energy beams shooting everywhere, flips and stuff like that. By the end of the game, when I'd acquired most all of Sora's additional combat abilities, there would be so much crap flying around the screen during battles that I couldn't ever really see what was going on. It's more a matter of personal taste, but I greatly preferred KHI's simplistic but satisfying system.