A marathon of cameos. A game made for kids. If you can get past that, a very entertaining ride.
That being said, the reason you'll BUY Kingdom Hearts II is because the game DOES have a story, it DOES have awsome gameplay and not to mention, the fanservice included into the package is not what the story and action revolves around. So if you feel shallow, or feel the game is shallow, then don't be ashamed to give into your guilty pleasure. You will be pleasently surprised.
The story of Kingdom Hearts is about Sora, Donald and Goofy thrust into an epic quest to stop the evil "Heartless" from smothering the light of Kingdom Hearts in darkness forever. Kigndom Hearts II picks up roughly one year after Kigndom Hearts and it's semi-sequel, Chain of Memories. Thankfully I didn't play Chain of Memories because I would have been sore about how it DIDN'T conclude the story, but I imagine if you were to play it, it would give you a leg up on the story that connects the titles. I was able to pick up enough from the synopsis and the introduction to get past it, so I didn't feel all that left out.
The plot thickens in Kingdom Hearts II with the PS2 introduction of Orginization XIII and the Nobodies. As you delve deeper into the story, you'll begin to realise you were a mere pawn in the middle of something so much more.
The difficulty of the game is fair. Square-Enix's games are usually more story-driven, and they like to make the game easy enough that you can enjoy that story. Of course, you have the option of choosing a harder difficulty right from the start, for gamers who like that kind of rush in the gameplay, but it's not so crazy you'll start bleeding from the forehead. The game will get more intense as you progress, but you should have a decent handle on the controls once you uncover the true conflict of the story.
The action which I emphasised was really awsome is standard hack' n' slash. The game is button-masharific, and a lot of the game can be beaten simply by spamming the [x] button. Kingdom Hearts 2 introduces a few new elements though.
The first, and most important are the "Reaction commands" where a green triangle will flash on an enemy and you have the option of performing a special attack or counter against them. This feature can help you do some grand-standing in some of the more fierce battles you'll encounter. It's not time-sensitive like the special combos in KH1 either, and even mashing the [triangle] button like crazy doesn't interrupt the combo. Hence, if you have a turbo controller, you can set Triangle to turbo and have no trouble performing reaction commands when the opportunity comes.
The second is the "Drive/Form" guage, building this up lets you spend drive points to "merge" with someone in your party in order to power up to a new form. You gain several forms throughout the game and each are powerful in their own way.
Graphics
Most of the game is stylized, but one look on the back of the box shows you it's almost photo-realistic in some parts. Each world has it's own visual style, as did all the disney movies, the only one that didn't actually follow any style was the photo-realistic "Port Royal" level (Pirates of the Carrabian). All characters are hand-animated, no motion capturing, which adds to the visual style, although I wish they would have used motion capture for Port Royal; the rest of the game didn't need it. AS FOR the lip-synch for the english dialogue, Square Enix DID NOT skip the effort. Every character's lip-synch matches the language they're speaking, and that's something we can all appreciate.
Sound
Music is awsome.
Voice acting is especially awsome, as it was unhindered by ADR techniques. The voice acting is true english, and many of the characters are voiced by their original actors. Almost ALL of them in fact. You can't actually look them up in the instruction booklet like you could in KH1, but they really do have almost everybody. James Woods returns as Hades, They have Auron's original english VA, and the characters that before had no voices, like Cloud, Squall, and yes, even Sephiroth, KICK ASS! I wish their part in the story was not so small, but even so, there's plenty of story to keep you occupied and happy.
Replay Value.
Erm... limited. The game is so long that a lot of it's value is put out there in the first play. Once you've beaten the game, you can try completing Jimminy's Journal, but Square's primary is story. The game doesn't do much to hinder your progress, and completing all the sidequests is not so much for an ultimate weapon, but for that extra little bit of story for you, the fans.
Conclusion
I said it right off the bat. This game is fanservice, and quite possibly a last ditch effort to breathe life into old characters, but the story of the game doesn't revolve around that fanservice, only enhanced by it. The actual story could work without any of the Final Fantasy or Disney characters, and is a very fun trip. Worth the money. Go. Buy. Now. If it's in my collection, you know somebody bought it. You want to be cool, right?