Well, it all comes down to opinions! FFXIII has some flaws but I think it has enough qualities to make it a really good game. There are aspects of the game that havent been done better yet, IMO. The obvious and already thoroughly discussed in other threats (and debated) are the character development and battle system. It's a universal opinion that the lack of mini games and side quests other than hunts is unfortunate.

Although I would like to point out that I am surprised at how complicated the weapon upgrading system is. It's much easier to screw up than X's and especially IX's. It is also crucial for post-game content. That being said, there are some superbeasts in XIII! I have to give the edge XIII over XII. I know that Yiazmat has 51 mil hp and I have fought him myself, but the fact that you can leave the battle and return makes the whole experience underwhelming. The boss felt like a chore instead of an epic battle. Slaying Adamatoise's in FFXIII is stuff of legend, however.