Killer App: Final Fantasy X was one of the biggest system sellers for the PlayStation 2, selling a total of 8.5 million units, and being the highest-selling game on the console that wasn't part of Grand Theft Auto or Gran Turismo. And as a result of the PS2 being the highest-selling console of all time, many people were introduced to the series through X, thus making it the equivalent of Final Fantasy VII for its own console generation.
No Export for You: The US didn't get the International version initially, only the PAL regions and Japan. This was not rectified for thirteen years, until the HD release, which has the International version's content. To be fair though, this is mostly Sony's fault, who has a policy stating that a game must have a certain percentage of new content before it can be re-released and the International version of Final Fantasy X doesn't meet the criteria, and this was an era before consoles were capable of receiving DLCs.
Tidus was originally supposed to be revealed as an unsent. However, Nomura saw a movie with a similar premise, and decided to swap Tidus' reveal with Auron. Tidus ends up being kind of similar to an Unsent, but his Cessation of Existence isn't something Yuna can control.
In the original draft, Tidus was going to be a plumber(!) who was also a member of a Japanese-style gang.
Each character's standard progression in the Sphere Grid is reminiscent of at least one Job Class from previous Final Fantasy games. Tidus is a Fighter-Time Mage combination, Yuna is a typical White Mage/Summoner, Lulu is a pure Black Mage, Wakka is an odd take on an Archer-Green Magenote given his status-inducing attacks fusion, Auron is a FFT-style Knight (with some stylistic and aesthetic references to the Samurai class), Kimahri is a cross between a Dragoon, a Blue Mage and a Red Mage, and Rikku is a wonderful blend between a Thief and a Chemist.