Simplistic battles and story are balanced by intriguing characters. One step behind the main series in every aspect.
You play Roxas -> Sora + X. Sora's nobody. Roxas has no memories of his original form other than bits and pieces and works for organization XIII, a group of nobodies that were strong enough to keep their original form and their memories (save for Roxas). Their goal is to complete Kingdom Hearts in order to obtain hearts of their own.
The game is built around a mission system set from a single starting point in Castle Oblivion. You will not travel to worlds and explore them as was the case in the previous installments. Stories in the worlds you visit are fragmented and not coherent. Each time you visit you are presented with a short intro and some minor story elements. The worlds are only there so you can swing your keyblade. The real story is comprised of short cinematics between the missions.
This is where the flaw in this game springs forward. The missions aren't boring. In fact they are quite fun to play 30 minutes before going to bed for instance. If it weren't for Roxas, Axel and Xion, the game would be a mindless offspring from the genre. The Kingdom Hearts series has always suffered from a battle system that was far too easy and required little more than button bashing. The battle system however is a step backwards from Kingdom Hearts 2. You only get to equip a single keyblade with limited options in terms of combos. The skill set is identical to the previous games with the addition of some small modifications to block, roll and such. New is "dash". An ability that allows Roxas to move horizontally in the air. Quite useful at times. In addition you do not visit any new worlds you haven't already visited in the first two games. There are some new enemies, but nothing exciting.
The story contains no depth and seems to have been created solely to bridge the gap between Sora and Roxas. Nothing new about neither Kingdom Hearts nor organization XIII. If you played Kingdom Hearts 2 you will already know all there is to know about this installment. The final fight is terribly easy and disappointing and the finale is stale and hurried.
Kingdom Hearts 359/2 days brings nothing new, not even new characters, and it's sole purpose is to give fans a brief glance at what happened to Roxas before the events of Kingdom Hearts 2. It's still a fun game with some gripping moments. Aside from that it's a rather forgettable game.