I honestly can't tell you what I liked about kingdom hearts except that I'm a loser for disney. |:
I get bored of the game really fast and I only really enjoyed Birth By Sleep since it was a little less... cutesy story wise. But I play them all anyway just because I need something mindless that'll take not too long to play.
But what dragged me in the first time was the battle system. It irritated me in the first one, but the 2nd one fixed it a little and made it a bit better.
Personally, I liked the light-hearted whimsy and simple story of a boy going on a magical adventure to travel to Nostalgic Disney worlds while trying to find his best friend, fight the bad guys, and save the girl. It didn't try to be too deep or dark but wasn't shallow and didn't talk down to its audience because it was a kids game and relied on the "Disney Magic" just enough, but with out relying on it completely to carry the game. Also with its original characters and worlds I think it could carry it self if it chose to so if you don't like the Disney aspect you have the option to enjoy it for the original material or the FF aspect.
Or at least that's how I felt about the first game. I love the series as a whole but I do feel that after the first game it did lose touch with its innocence and whimsy and occasionally went down the dreaded "this is for kids so it's okay to be stupid" route but that's a whole other can of worms.
I'm a huge Disney buff, so that was the hook for me. In fact, I hadn't played any FF games so I didn't recognize a single FF character. Disney was literally the only appeal. Not to mention, the opening of the game was just dazzling. emotion_kirakira