Well, the mark of a great game is how much controversy it stirs up in the fan base. Not entirely related, but I've just noticed lots of heated debate on FFXIII's awesome or fail factor on the internet.

Post-game is just there for anyone who isn't done with the world yet, which is an interesting twist on an old mechanic, leaving just about all the side-quests for only after the game is beaten that is. Obviously the hardest challenges of the game rest in this part of the game (it should go without saying that the initial onset of the sidequests post-13 is pretty easy, getting rank 5's can be annoying at times though).

Honestly, I'd play through the whole game again just for the cutscenes if my roommate wasn't already going to play through it.