I feel all fluffy inside reading that so many people actually love DDD just like I do, ironically exactly for the reasons you listed as negative xD
The challenging gameplay made you focus on Keyblade, commands and Dream Eaters simultaneously and, while the first two things seem of main importance, it's actually the Dream Eaters that add action to the game. Sure, there is Flowmotion which is hella cool especially with 3D activated and playing around with the camera, but it's because of the Dream Eaters we get new commands, items, abilities. In most cases, that makes the player happy to have them, or to have discovered them (we know we push our luck sometimes to get a new, unknown Dream Eater~).
In the first run you don't know what to expect from a Dream Eater, there's equal chances of feeling disappointment in one and absolute joy because of the other. You end up having a preference for a few, because of their abilities or their design, or both, and... I must say, personally, the first thing I do when I load DDD now is to pet the 2 Dream Eaters I got attached to most, a Me Me Bunny and Keeba Tiger. I will NEVER forget how my beloved Keeba Tiger saved my ass during the Young Xehanort battle, healing me just in time to finish YX off, and casting Dark Firaga after to help my final attack.
Yeah, it's this kind of feeling with DDD, if you take the game properly and openly, even with its difficulty that may become tedious to some. To me, this difficulty was what kept me interested till the end, actually, and made me want to master all of the game's features, and to maintaining my interest the full-of-cliffhangers wtf story contributed a lot. That was good. And the areas felt gigantic, just think of the Hockomonkey battle on Sora's side, now THAT was a huge battlefield with plenty of space to run, hide, or get caught! Or the MAJESTIC areas of Symphony of Sorcery, I personally felt like I enjoyed every single step Sora or Riku made in that beautiful world. Not to mention Prankster's Paradise on Sora's side. The way the big areas, even empty of NPCs, encouraged you to explore just by how open and endless they seemed sometimes, now that's true art.
I had the immense pleasure to complete the game 100%, I haven't felt so satisfied since... man, I don't even know. The only thing I didn't get was the MAX Affinity with all your Dream Eaters trophy, that IS too much, maybe I'll complete that in... some years! But that's not my priority, I started a Critical Level 1 run on DDD and it's a KILLER in the beginning, and, strange as it may sound to most players, I adore it, fantastic challenge and skill training.
I know the others said most of this before, but still I'm here to prove that from one more perspective, the one that belongs to me~
Even with my positive opinion on DDD, you are not alone, there are KH fans that dislike DDD, it's only natural that there are people who like it and people who don't. DDD is a hard game, both by itself and compared to the other KH games. Depending on how you like to play your games, that will make it either an exciting pleasure to play or a difficult rather saddening thing to get over with.
My friends looked at me playing BBS on PSP in class one day and they said what I did seemed very difficult, when, in fact, BBS is so simple. Thinking about it a different way, what would they think of DDD if BBS seemed hard? It really depends on each person, that's what difficulty levels are for after all, so that anyone can at least try