The Lore. I absolutely love the lore. You can tell the devs have put a LOT of effort into building the lore up in this world. They never ever just add a mob to just add it. Everything has a story.
Even the trash mobs and their names have a tie in to either our real world lore or the lore of the final fantasy series. Absolutely amazing.
This gets me hooked every time
Also, the music. Some of the music is just so beautiful. And the piano accompaniments...especially in Shor Khai. Wow.