The greatness of SE combined with Disney make a game that has an awesome plot and is really fun to play!
What's really original is that you start the game as Roxas, who appears as a mystery in the beginning. Later in the game we find out who Roxas really is.
Roxas finds Sora who was left asleep in the end of Kingdom Hearts:Chain of Memories. Sora sets out to find his friend Riku. Along the way he meets a group called "organization XIII, the vilains of the game, who he defeats in the end.
As regards the gameplay, it's true that you can get through the game by button-mashing, but you can enjoy it more if you don't. There are a lot of different tactics that can be used in battle. Also the difficlty level is neither too low or too high, but you can always choose to play in Beginner or Proud Mode. Lastly, then main story isn't too long but there are also a lot of side quests that can make the game last really long.
The only game I consider better than this is its Final Mix version.