Aluminum Christmas Trees: "Sea Salt Ice Cream". A lot of people in North America and Europe thought that it wouldn't possibly exist because the salt would lower the freezing point of the water, preventing the ice cream from solidifying. Or just that nobody would think salty ice cream would be good anyway. Actually, it's sold at Tokyo Disneysea - and has been before Kingdom Hearts 2.
Author Appeal: The reason it was worked in. It was also worked in because he imagined what the characters would eat over summer vacation.
Executive Meddling: According to Tetsuya Nomura, he had originally planned on only including characters from Final Fantasy titles he personally designed. He eventually caved in to the urging of the staff, which is how Setzer and Vivi were added to the game. The gamble perhaps didn't pay off as well as they would've hoped; Setzer, in particular, was criticized for heavily deviating from his original incarnation's personality, moreso than was par for the course in KH.
Fan Nickname: Before his name was revealed, fans took to calling Roxas the Blond-Haired Kid (BHK).
To those trying to guess the plot of the sequel from the trailer, Roxas was often called Dual-Wielding Unknown, while Riku was called Blindfolded Unknown.
Oddly enough the trailer isn't even part of the game; it's a flashback that takes place right before it. It is, however, the final battle in Days.
Don't forget "Orgy XIII".
Fans also commonly refer to Xemnas as "Mansex" - an anagram of his name.
Chip Cheezum and friends popularized "Dogstreet" for Pence, named after the writing on his shirt.
Fandom Nod: In addition to all the Disney/Final Fantasy references, there is a literal fandom nod during Cloud and Leon's snark-fest in the sequel. As they do their Back-to-Back Badasses moment, it echoes the rivalry between fans of their respective games.
Corey Burton provided Sark's voice because David Warner, his original actor, was unavailable at the time. Burton previously voiced Shockwave in Transformers, whose voice was inspired by Warner's performance as Sark.