Contributed by Shadowmane
Cloud's Buster Sword was originally intended for use by Tidus but was removed before the game was released. It is still in the game's code, though, and using hacking tools, it can be obtained and used in the final version.
Lulu's dress in Final Fantasy X was designed by Tetsuya Nomura specifically to see if the graphic designers could keep the exact number of belts in the right places every time. This is why many cutscenes with her show Lulu from the waist up or from behind.
In Tidus's early development, he originally had darker colored hair. It was changed, most likely because his name means "Sun" in Okinawan, and the developers wanted the color of his hair to reflect that.
Character designer Tetsuya Nomura has said that the South Pacific, Thailand and Japan are major influences on the culture and geographical design of Spira, particularly in the case of the southern islands of Besaid and Kilika.
Lulu's starting weapon, a Moogle Doll, is the only appearance of a Moogle in the game, a massive difference from all previous titles in the series. Her other weapons are references to previous characters in the series, including her final weapon of the Onion Knights from Final Fantasy III.
Final Fantasy X was the first core Final Fantasy title that wasn't entirely composed by Nobuo Uematsu. The score was also created by Masashi Hamauzu and Junya Nakano.
At the end of the game Yuna will proclaim to Tidus that she loves him. However in the Japanese version Yuna will simply thank him. This change was to make up for cultural differences.
Tidus is never actually directly referred to by his name in audible dialogue. People will refer to him with impersonal pronouns. This is due to the player being able to name Tidus at the start of the game, and this being Square Enix's first voiced Final Fantasy game, they weren't sure how to handle a variable name. Tidus' name pronunciation has been debated, but was confirmed to be pronounced "Tee-Dus" by James Arnold Taylor, the voice actor for Tidus.
There is an unused female version of somebody humming the Hymm of the Fayth, as hummed by Tidus during the game. It's unknown who the voice could be.