KH2 improves upon the first Kingdom Hearts and goes beyond that to offer an amazing experience.
The story on the other hand starts out confusing, and if you haven't played Chain of Memories (the TRUE sequel of KH) then you'll have a hard time catching up - even when the game shows you flashback cut scenes of the last Kingdom Hearts. Instead of playing as Sora you start out at his Nobody Roxas who lives in Twilight town with his three friends. Right off the bat you'll be doing mundane tasks such as searching for the missing "___" word, fighting FFVIII characters with a toy baseball bat, and doing several job tasks to earn enough munny to go to the beach. Roxas' prologue takes at least 2 hours to pass before the main title even appears and is more of a long drawn out tutorial than anything and doesn't give you much of an insight of the Kingdom Hearts world or the Organization XIII if you haven't already played the other games in the series (and shame on you if you didn't!). Fortunately, those who are patient will be rewarded as the main title fades away, Sora awakens, and the real game begins. Now you must team up with Donald and Goofy to defeat Organization XIII and the Dusks, find Riku and King Mickey, and save the Disney universe yet again from the Heartless.
As you begin the game with Sora you'll notice the battle system has been ratified. Not too much, but just enough to make the game have it's own new unique feel. Stringing combos is now flawless thanks to jumping being less lead weighted as it was in the first Kingdom Hearts, which made plat forming a hassle. Selecting and using various magic is the same as it was in the first KH, but in this game the MP bar replenishes slowly after depleting. This might make the game sound easy, since you can continually heal yourself and cast magic, but since a lot of the magic in this game has such poor reach I ended up not using any magic besides healing magic throughout the whole game. Fortunately, Sora also has a new feature in his arsenal that's more powerful: Drive Forms. While in this mode Sora wields two key blades and can unleash a multitude of attacks until the Drive Gauge depletes. Also, when in this form, Sora loses a teammate for the length of the Drive form, but since the forms are so useful it's a small sacrifice for only a couple of seconds. These forms can also level up, so using them often is essential to unlocking new forms and gaining new abilities.
Difficulty wise, KH2 isn't all that challenging in the first two modes, so I really approve of starting the game out on Proud Mode if you're a regular at action games. Even if you do blast through the game, KH 2 is a blast. There's always something new around the corner to keep you entertained, and teaming up with Final Fantasy characters while knocking the snot out of Heartless/Dusks never gets old. If that doesn't thrill you, then maybe the challenging boss battles might, which take precise blocking and careful planning to defeat. KH2 may not be perfect gameplay wise and become tedious at points, but it's still good fun.
If anything at all, KH2 provides great nostalgia to Disney fans. You'll journey to new worlds inspired by such movies as The Lion King, Pirates of the Caribbean, Mulan, and fan favorite Tron. If you fear that the first play through of these worlds is short, don't worry - you'll be coming back to these world's a second time to make sure things are in order and also to advance the plotline. Old worlds return too with new places to explore like Christmas Town in the Halloween Town world for example. Though it's nice to visit these worlds again, some of them shouldn't have come back. As much as I like Agrabah from Aladdin, the world did run into same-y story plotline from the last game, and you even battle Jafar - again - in the same form! And let's not even bring up the Little Mermaid world with it's new musical rhythm game…
Sure, Kingdom Hearts 2 has a confusing plotline and the returning worlds may be mediocre, but for those who liked the original and have already played and/or understand COM or 358/2 Days' plotline, this game is a MUST play. Sequels don't get any better than this.