Level design in KH1 was really cool. I liked how it teased the player. Hey, here's this treasure chest in the middle of this platform. Let's see if you can get it. So you try your hardest to get it, and can't dammit, because you don't have the right move to pull it off. Fast forward to 30 hours later and you can FLY and suddenly you can get to that damn treasure chest. It was really rewarding and it's why it's my favorite Kingdom Hearts game. The platforming was flawed, but it was still a blast. Kingdom Hearts 2 deduced everything that was great about how KH1 laid its levels out and became a boring linear masher without a hint of exploration that the original had. Nothing in KH2 tops the sheer grand scale of Hollow Bastion in the original game. What a fantastic locale.
I doubt Birth By Sleep offers the same experience either, going by Rah's post. I loved how the game rewarded discovery. There's a trinity symbol over at the Coliseum the entire game that you can't access until end game. It always drove me mad.
Charming story, charming characters, fun battle system, semi-challenging difficulty, wonderful sense of exploration. Kingdom Hearts 1 is a classic game. Chain of Memories is great, too. But after that Kingdom Hearts just lost me.