This is one of my favorite games ever and here's why:
1) Plot. This game's plot is amazingly compelling for a little black cartridge. Sora's storyline leaves something to be desired, and is sometimes tedious. However, the secret mode that is opened after you beat the game once has a storyline that is much cooler, and doesn't drag as much. The characters are well developed, and the plot is intriguing. However, if you don't like vague plots that require resolution in the newly released KH2, you might not like this storyline, considering it leaves a lot unsaid. However, the plot points it does add to the story are very interesting and foreshadow KH2 in a most excellent way.
2) Combat system. Many professional reviews have called this system "clunky" but I loved it. Many said you had to "spend time looking through your deck to make combos, so it was ineffectual and it always turned into button mash", but this is just plain untrue. The card system sets you up with a deck of cards that allow you to perform attacks and magic and such. However, you can also load three cards to perform an attack. The trick is that your deck is always in the same order when you begin a battle. That is to say, you can SET UP combos in your deck and they'll always be in that order, which is highly useful and ends up looking really cool. You can also set up decks with themes. For example, take really fast swing cards that let you attack more quickly, then combine it with the special card that increases attack speed and you've got a lightning fast deck for supreme enemybashing. Or, set up a string of magical combos to destroy the enemy before they even get a chance to retaliate. The number of combinations is really quite impressive and the options leave a lot of room for customization. The element of collecting cards is also really fun, but that's really a taste thing.
3) Technical aspects. I'm not big on graphics or sound, but this game seriously delivers. The graphics are crisp, and battles look great. The exploration graphics are also rich, although highly repetitive. If you can't stand environments that "all look the same", this game might drive you crazy. The rooms in each world lack diversity and all are fairly identical, but as long as that doesn't bother you, you're good.
4) Tilt. GameSpot uses this so why can't I? This game just plain rocks. Everything fits together nicely and it feels like a gaming experience that you've never really had before. Everyone should play this game, it's the only must have on GBA in my opinion, and it's story is only beaten by the recent FFIV release.