My favorite FF's are 6, 10 and Tactics. The only ones I truly can't stand are 8 and 12. So far, I love 13. The level system is a mix of the Job system and 10's Sphere Grid. You earn Crystalsomthingorother Points based on how well you do in battle, and spend those to get more HP, new skills for certain classes, STR, etc. I didn't have everyone in the party and capable of swapping active members until well into Disc 2, as the stories branch from here to there until everyone comes back together. The battle system is a mix of 12 and a whole new idea. You have a segmented gauge that determines the number of actions you are allowed to take, and it recharges after said actions.
The coolest new thing is the Paradigm system. Every character has certain classes they are able to use (more opening as you progress) at different points in the game. The main character, Lightning, for example, has Commando, Ravager and Medic at the point I'm at. There's also Synergists, Sentinels and Saboteurs. Each class has it's own benefits and abilities. You set Paradigms to swap between class setups in battle. The usual Commando, Ravager, Ravager set up is high damage output, but no debuff, buff or healing capability. So you might want to swap to another Paradigm in which you have a Medic, or maybe one with a Saboteur and Synergist. It's easily the most in-depth combat has been in a FF game, with the exception of Tactics.