The world gives a nice vertical element to Kingdom Hearts, even if Sora seems a little slower than normal. The ocean world breaks up the style of gameplay enough to give a new and interesting gameplay. But it seems like people don't like it much, why is that?
The fact that they change the swimming/flying controls every game pisses me off. KH1 had a fine system square to go down, circle to go up. It made sense. Then in KH2 they had to go and change it to right stick for up/down. And finally in KH3 they change it to the triggers for up/down. I preferred the KH1 controls.