Storyline---Awesome. The whole light and darkness thing had me hooked. I wish it was a bit deeper and darker, more like Final Fantasy, but it's still awesome just the way it is!
Characters---All the original KH characters were pretty cool. My favorites are Roxas, Sora, Kairi, Riku, Namine, Olette, and all the Org. They made [some of] the FF characters look dumb, but I was just happy they were in there. Sephiroth made my day.
Bosses/Enemies--- I liked the concept of the hero(s) fighting darkness...it was cool. The nobodies were sooo awesome, all of them. Especially Org. XIII. I also liked the real time fighting style, where you can actually jump around and dodge things. Worthwhile bosses: Sephiroth, The Phantom, Kurt Zisa, and Org. XIII battles.
Worlds---The World That Never Was and Radiant Garden blew me away. The designs were different in each world, which made it nice to look at.
Weapons---OblivionXOathkeeper and Way to the Dawn are my favs. The keyblades in general were a unique idea that i loved. The other weapons were just ok.
Minigames---I was rather disappointed with this category(sadly). I just didn't like them.
Abilities---Very nice job. Dodge roll, Guard, and the combos really helped me out of some sticky situations. Some of the abilities were more useful than others, but that's what made it good.
Music---I own the KH soundtrack. It's amazing. The music is unbelievable(ly great!).