In my opinion Final Fantasy XIII is much better than Final Fantasy XII. There are lots of things to appreciate but most are taken for granted. I'll give you two examples:

1. Inertia: If you run and stop all of a sudden, then the character will slightly move forward due to inertia. I like the physics of this game, it makes it look more realistic.

2. Talking: To talk to NPCs, now you simply go infront of them with no need to press the "X" button. This way it's more natural.

If there's one thing I didn't like, it's the linearity. You rarely have the chance to decide which path to take. Hopefully this will be resolved in Final Fantasy Versus XIII with the re-introduction of the World Map.