It's very...different. I can't even really compare it to XIII and XIII-2 from a gameplay perspective.
I only have about 8 hours so far, so I can't make any final conclusions, but my thoughts are:
1. You're heavily rewarded for exploration and doing side quests. And by heavily rewarded, I mean "the majority of your stat boosts come from completing side quests, so it's very important/imperative to do so." I love this, personally. I can see others disliking it.
2. I'm adoring the battle system. I went through a bit of a wave with it. I liked it when I played the demo, but was underwhelmed at first when I started the game. By the time I got about halfway through the first major area I started picking up on how things are meant to be done and started realizing it's surprisingly impressive. Timing is quite important, such as interrupting attacks, attacking while vulnerable, and so on. You can even cut off body parts if you time things right.
The Paradigm system is only here as a way to change your command set. You have 3 customizable ones in battle with you. Each Schema (Read: costume) has different base stats attacked to it, making it more suitable for HP, MP, magic, attack, defense, balanced, so on.
Fair warning: The battles start out pretty slow until you start leveling your stats via quests and synthesizing your skills to strengthen them. You also really, really need to exploit enemy vulnerabilities, or battles will take forever. For example, one machine enemy is really only vulnerable when he's charging a certain attack.
3. Lightning is really offputting during the main story, to be honest. I'm relatively neutral towards her usually, but I found myself, for example, empathizing a lot more with Noel during his events. She is, however, a lot more likable during side quests where she is less harsh and more expressive and shows a softer side. Actually, thinking on it, she really is like Cloud in this regard.
4. I'm finding the music to be the weakest in the series so far, but I did like that one remix of Blinded by the Light, so maybe it'll improve. I haven't heard a lot of it, yet.
On an unrelated note -
I love how I can run around in whatever costume I like. I mentioned it above, but I've been running around in Aerith's dress because it looks nice. It's a bit funny hearing people comment on her outfit choice, from the flowers, to claiming she's a white mage, and so on. It also changes her post-battle quotes.
One generic NPC is a bit creepy and says "Cute bow!" as you pass. I don't think it's meant to be as disturbing as it sounds in-game, but yeah, if someone said that to me in real life I'd sort of inch away.