The lack of character development in XII really dulled the experience for me. Hell most of the characters didn't even need to BE there. Vaan: Just a tag along ~Penelo: Tagging along with Vaan ~Fran: Tagging along with balthier.
Only Baltheir, Basch and Ashe had anything to really contribute to the story. Which is truly a shame.
While I really enjoy the way-too-long dungeons (no really I do) it's very easy to forget why you're even there before you even reach the half-way point.
Granted I always forget the plot exists when I even play that game now, not because I know what happens... but because the game makes you forget it so easily it's hard to get back into it.
Overall the gameplay and combat keeps it in my favorites, not to mention the stuff to do post-game and all the sidequests/areas to explore.