Kingdom Hearts plot basically becoming Final Fantasty-esque is well established because it is just as unncessarily confusing and overtly complex as any recent titles in that series now.
Well, yeah, but that was mostly post-KH2. When they started doing prequel **** and all that.
KH2 was pretty basic, and had a sensible (and decent) lead-in via Chain of Memories. You had your initial obvious enemies, but it was pretty obvious that Org 13 was the true threat all along.
I still say my biggest gripe about the game as a whole was not enough Riku, though. I thought his rivalry/friendship/jealousy undertones in KH1 were pretty good, and I was a sucker for the redemption arc angle, but most of that got booted into DDD.
Anyway, what I liked about Kingdom Hearts 2 was that it introduced new characters, that had some actual duality added unto them, which was a welcome addition, alongside the expected Disney content we got as well. More Nightmare Before Christmas is always good in my book.
Haloween Town was always my favorite world. It was right at that sweet spot in the first game of being both challenging but also the point at which you were really unlocking all your powers for the feeling of a good fight. Plus, it was an adventure fighting through everything. In KH2 it was also good.
I think a flaw of KH2 was that the Maleficient arc dropped hard, though. The Darkspawn were really just aimless filler for the most part, getting in the way of the real fight.
As for duality... well, like I said. Not enough Riku.