I'm playing it for the third time right now, and just can't believe how much fun it still is. I have been persuaded to start a new game when I remember how much fun the story is for the first part of the game. Everything before the point where you head to Kerwon is just like pure fun to me. Also, it's fun to try to explore as far as you can in the world while progressing the story as little as possible.
What really makes the game totally replayable for me is just how much fun it is to try to get characters really strong with various weapon/armor combinations. FFXII got this system perfect.