Well, I like the Job System for classes because it gives you a sort of reward for working in a given job. It's realistic that you get better at certain things when you work at them over and over again. As in XI where using a certain magic more than another would give you bonuses in that area of magic.

I also liked XII in the way that you could seemingly work on whatever you wanted at the same time. But really, it's just sort of a gimped version of XI's abilities, I say.