FFX is a good game.

Unlike FFVIII and IX, it didn't throw the majority of the party under a bus. It gave each character their own arc and conflict to overcome and they remained relevant from start to finish. It also avoids the common flaw of FF games - the flaw of "why the hell are they even in the party?" - by having a common objective from the start of the game.

The voice-acting is passable and even good in some points. It's not up to Metal Gear Solid 2 or Xenosaga to be certain but it's far from awful. The direction is the problem.

The game does a fabulous job with atmosphere, establishing this bright and colorful world that is simultaneously also the darkest and most depressing world ever seen in an FF game.

Seymour was a good villain. He didn't have the menace of most other FF baddies but he obviously wasn't intended to. When there's a giant doomwhale that might attack, it's hard to find anything else intimidating.
No, Seymour was intended as an evil counterpart to both Yuna and the party as a whole. His overall goal or motivation was in essence the same as the party's - liberation from the Spiral of Death.

I felt the "big important scenes" were also executed absolutely perfectly. Yuna is gonna die might have been obvious as hell but that didn't make the reveal iN Home any less emotionally charged.
Facing down and rejecting Yunalesca was also one of the best scenes ever in the franchise.

The soundtrack was fantastic and contains some of the best pieces ever heard in the series.

There is no defending the costumes though... Well, I defend Rikku's because I like staring at her.