The most unique gameplay experience you would ever have. Period.
YAYS
Game improved every aspect of previous version
NAYS
Some FF characters got butchered; game still eventually ends
The first Kingdom Hearts introduced the concept that not every Disney game has to suck beyond comprehension, and that Square is capable of besting itself by combining an intriguing story with already well-established characters. Although it was slightly light on story, the first KH proved to be a great success due to its memorable cast of characters and gameplay variety.
KH2 is the direct sequel of the first KH and KH: Chain of Memories released on the GBA. If you haven't played any, then it is best that you play them since it will make you appreciate KH2's story and inside jokes more. Regardless, Square has done a good job in re-introducing the story for the new comers as well as for those who need a memory update. You spend the first few hours playing as Roxas, a teenage (why Square is addicted to teens is beyond me) resident of Twilight Town, who is having some odd flashbacks and dreams that feature some mysterious characters he never met. These characters are Sora, Donald and Goofy, as well as some others, from the previous games. Roxas has no idea of what is going on, but sooner or later things get disturbed and you immediately notice that what is in front of you is a grand game. You get attacked by a new entity called Nobodies, and, without giving too much of the intro story, sooner or later you would be playing as Sora again.
As Sora, you will be doing what you learned to do best in KH - defeat countless of Heartless, some Nobodies, and save the day in various Disney themed worlds. This time though the ride is much more fun, interesting and intense. Some Disney worlds have returned from the first KH, like the Coliseum and Agrabah (as well as Atlantica in some odd musical form), while some others came new (like the Pride Lands and Castle Disney). This time though, sealing each world and moving on is not the only thing you would be doing until the end, like you did in KH1. Rather, midway through the game when the identity of the Nobodies is revealed as well as the game's main antagonists, Organization XIII, show up, you would be re-visiting all the worlds again for a continued story line tied to the main story. Which is good, since in the first game each world had a self contained story that contributed very little to the overarching theme. This time around everything makes more sense, especially that the story has significantly grown more complex.
During your quest, you fend off against the Heartless, Nobodies and the villains from Disney characters with your trusty Keyblade. This time around though, there is a much larger variety of Keyblades at your disposal, each with its own strengths and weaknesses. If you're confused on which Keyblade to equip, you can now fight with two Keyblades, and this is one of the more significant introductions to the gameplay. Sora can now, using a Drive command, fuse his power with either Donald or Goofy (or both of them) to change into a different "form". The forms are a powered up version of Sora, where mostly he would be using two Keyblades and has access to different abilities and devastating combos, making fights with bosses and the members of the Organization look like the fights in Advent Children. They are incredibly fun to play in and more often than not would save you out of a deadly situation, although in some cases it is not a good option. Each form has its different abilities as well, so one form may work better in a situation than another. During the form, your Drive Gauge is drained, and when it reaches zero you will revert back to your normal form. Building up the drive gauge is easy though, by simply hitting enemies, being hit or by collecting special Drive orbs. It takes some few minutes to fill up the Drive Gauge, although most likely you won't be using your different forms except in hopeless situations.
One other significant change is how MP is handled. In the previous game, each magic cost some MP and you had to use Ethers or collect MP orbs to replenish your MP. In KH2 however, while the basic idea is the same, when you use Cure however it completely uses up all your MP. Although it is on its surface a disturbing fact, don't worry, your MP will recharge upon depletion (it will take time though) and it is a very good move on Square's side as it makes you more considerate about taking damage and your fighting strategy since you know you won't be able to cure for a while and would have to rely on your quick fingers and other various potions. Another introduction is Limits, where you can perform a Limit Break on your own or with another party member. Summons also make a return, although with all the above-mentioned list of things you can do, most likely you won't be using summons. All these abilities consume either MP or from your Drive gauge, so it is balance and you would always be having an option to use for your battle strategy.
Another, most significant addition to the game is the Reaction Command. Although it is mostly used in fights, there are various reaction commands that pop up for you to interact with your surroundings while investigating a town or area. In battles however, the reaction commands truly shine. In specific situations when encountering enemies, with all circumstances right, a reaction command will pop up briefly. If you succeed in executing, you can pull off a devastating attack or counterattack, or in some cases to evade fatal enemy hits. It is most prominent in boss fights though, and there are some boss fights which mostly rely on reaction commands and your quick reflexes rather than on your Keyblade skills.
Speaking of boss fights, there are tons, and tons, and tons of boss fights. Each world has two, and you get to fight most members of Organization XIII and each one is as entertaining and challenging as the one before. The sheer amount of variety and creativity put into the bosses is remarkable, and while all eventually succumb to your brutal Keyblade combos, the experience in the variety and the execution of the reaction commands ups the fun factor exponentially. You'll never look at an action RPG in the same way before.
Adding more good stuff to the game is vastly improving the gummi ship levels. They are now, literally, like Panzer Dragoon, just as exciting and hectic. Lots of enemy variety, items to collect, and even ranks and objectives makes the gummi ships a great mini game to pursue when you want to have a break from the main action. The ship-building tool is quite deep and highly customizable, but thankfully there is a well-detailed step by step tutorial to help you along the way.
With all these vast improvements, however, there are some things that were not done right, even if they are dwarfed by the sheer number of good stuff. First off, there are many other Disney characters that did not make an appearance while others appeared twice. Although, from a commercial viewpoint, it is understandable that the more popular ones are present, it seems a bit odd that other themes could have been added in addition or in place of other themes. The second negative point is that some Final Fantasy characters are given terrible makeovers, roles and/or lines, and if they happen to be your favorite, you're going to voodoo Square for it. For instance, Vivi sounds terrible (although he is still as cute and fun). The lady trio from FFX-2 (especially Yuna) are reduced to flying midgets with annoying voices. The great Seifer has become a gender confused teenager. Aeris gets the vote for the worst voiced and worst dressed FF character in KH, and given her popularity, Square is going to be getting a lot of hate mail. You will also be surprised why some FF villains don't make an appearance, like Edea, especially that she is a sorceress and would fit well in the story. Sephiroth however shows up yet again for you to try to defeat as an option (if impossible) boss. The good part though is that in one point in the game you would side with all of them and fight alongside them, which is really cool.
With all that said, the game is well presented artistically, aesthetically and accompanied by a great musical score. There are tons of special effects and eye candy scenes. Each world is rendered with great detail and the artwork is simply superb. The animation is the best part, though, and Square should learn to make their characters in future FF games animate just as fluidly. There is lots of body language and movement, and it is apparent that high production values were put into the game. The music is great from beginning to end, with some recycled music notes and some brand new and really great ones (especially when it comes to the Organization).
The game is superb, both in concept, story and execution, and it is endearing the maximum level. The cast of characters is memorable, and how they all intertwine into the story makes the game more likeable and closer to you that you feel you are attached to the game more than any other game you played. Both Disney and Square should have learned from this collaboration, and it should be set as an example for other games to follow. This is one of the best RPGs I played, and without a doubt the most fun I had for a long while. The only really sad part about the game is that it ends, and although KH3 is around the corner, I don't know where the story would go, and I pray that Square isn't eaten by its own greed and would start ruining the franchise.
Play the game. On Proud Mode. You won't regret it.