Kathryn Beaumont, who voiced Alice and Wendy in their respective films back in the 50s, returned to do both roles despite being in her mid-sixties (she's also one of only a few actors/actresses who worked with Walt Disney directly to be involved with the Kingdom Hearts games as well; John Fiedler, who voiced Piglet, did the role in the games as well until his death).
Jodi Benson at age 31 once again voiced the 16-year-old Ariel.
Development Gag: Rikku from Final Fantasy X was going to be included, but she was replaced with Yuffie. As a nod to this, Yuffie's outfit is the one given to Rikku in concept art.
The Other Darrin: Several Disney characters from older films had different voice actors, due to their original actors being deceased. Tress McNeille replaces Verna Felton as the Queen of Hearts. Susanne Blakeslee replaces Eleanor Audley as Maleficent. Corey Burton instead of Joseph Kearns as the Doorknob, and Hans Conried as Captain Hook. Christopher Steele instead of Bobby Driscoll as Peter Pan etc.
Sean Astin does the voice of Hercules, who was voiced by Tate Donovan in his film. Donovan did the voice in the second game.
Kevin Michael Richardson replaces Samuel E. Wright as the voice of Sebastian, the only time the character has been voiced by someone other than Wright.
Due to the characters being much younger than their canon counterparts, Tidus and Wakka have different voice actors. Tidus is now voiced by Sean Fleming instead of James Arnold Taylor, while Wakka is voiced by Dee Bradley Baker instead of John DiMaggio.
There was planned to be one more battle against Ansem-possessed Riku at the End of the World. It was very nearly completed but Dummied Out in the final release.
The original scenario involved Sora saving the Princesses of Heart with the assistance of their respective princes.
Kairi's role in Hollow Bastion was going to be larger.
The recurring Final Fantasy Summon Bahamut was going to be featured.
The original concept for the game had Mickey as the main character, with Donald and Goofy helping out. The development team eventually became divided on whether he or Donald got to be the main character, and so they compromised by creating Sora.
Sora's original design was more lion-like and wielded a chainsaw sword. This was changed to a giant key to be less violent.
Many early screenshots for the game showed Donald and Goofy wearing their classic outfits from the original Disney shorts. This was changed to the current attire they wear in the game, presumably to give them more adventurous appearances, and they can only be seen wearing their classic attire in the game's epilogue.
Chernabog was going to be the final boss, being the embodiment of all darkness. This was changed for various reasons. Some concept art shows that the battle would take place at an unknown castle.
Some concept art showed Sora wearing a loincloth in the Deep Jungle and a world icon for a Toy Story world.
A world based on The Lion King was planned but they did not have the technology necessary to turn Sora into a lion, so instead Simba only appears as a summon. The world was eventually featured in Kingdom Hearts II.
A world based off of the movie Toy Story was considered during development, but never picked up traction.
"Destati" is apparently considered cursed by the production team due to odd incidents—the worst being the entire building losing power—occurring whenever the vocal track was inserted into a piece of background music.
A Bonus Boss is named after a fan who won a contest. 'Kurt Zisa' appears if you try to fly to the Cave of Wonders from Aladdin's house after visiting Hollow Bastion.