Its battle system is horrible. So slow it makes normal battles bore people to death and so simple that it lacks any kind of strategic depth. Plus its form of "limit breaks" is the worst in the series. IX is one of the worst playing entries in the series. Even IV (a FF game I absolutely hate) is better.
The trance system was kind of annoying, but I think the majority of the "slowness" is just the PS1 having trouble loading animations with a reasonable amount of speed. I liked the four-character party and the distinct roles every character played.
Vivi is the saving grace of the cast, otherwise it's not that much more special (Zidane is a boring main character and a total copy-paste job of Cloud story-wise). And Yuna is a far more interesting female protagonist than any of them from IX.
I like Zidane a hell of a lot more than whiny Tidus, and Yuna is one of the most boring, uninteresting lead characters in the entire franchise. It may have just been her completely passionless English voice actress that did it, but I used to refer to Yuna as "The Human Doormat" among my RPG-fan friends back in the day. Zero spine, zero charisma. Just a vapid, soulless void of a character.
The whimsy and the energy of the characters in IX was just so much more fun to me than the relatively serious cast in X, which was then dragged down by Rikku's hyperactive catgirl nonsense. Auron was the only character in the FFX cast that I actually liked. Well, Jecht too, but he wasn't a party member.
I've never liked IX's OST that much. It has a few stand out pieces but otherwise it's forgettable. X's with Hamauzu is much more memorable.
Honestly, I kind of hate Hamauzu's style. It's just so quiet and dull, everything just blends into the background. I like melodically-driven music, and Hamauzu's stuff is way, way too ambient by comparison. I literally don't remember anything out of FFX's soundtrack except for the very obvious Uematsu songs. Meanwhile, IX has a lot more songs that stuck with me, like Melodies of Life, You Are Not Alone, Vamo 'alla Flamenco, Hunter's Chance and Dark Messenger.
Speed and at least some level of strategy, which make its battles infinitely more entertaining than IX's slow struggle.
I guess we're just going to have to agree to disagree. I just brute forced through the entire game with Aeons.