The battle system wasn't fundamentally changed, but the entire philosophy behind it is. It's so much more fun to be able to chose and manipulate your own stats, monsters, etc. The problem is, it's way too easy to break that game. Being able to level up from quests limited grinding but also made it really easy to max out your crystarium compared to the mountains of effort needed in XIII. You had more choices in how you wanted to build your third member, but at the same time things like syns and sabs lost their relevance except in a select few cases.
It boils down to what kind of gameplay you prefer. The only things I can consider a real upgrade rather than just a shift in styles is getting rid of that first paradigm animation shift, chosing AI targeting, and saving your paradigm stacks.
I don't remember who brought up this point first, I think it was Splintered in the XIII-2 thread? Excellent observation btw.
I think a lot of people did. The only arguments I've seen against Noel is that he's too much of a "safe" character but, with only two protagonists there's nothing wrong with that. Especially if you have a wacky story, it's better to have grounded characters anyway. Zack's a safe character too, imagine playing Crisis Core when your only character is a douchebag.
The characters are completely different because, well, the philosophy behind the characters are different too. I always maintain that XIII's characters were suppose to be slightly unlikable when you first meet them. Meanwhile their call to the Hero's Journey is confusing, frustrating, they have no freedom whatso ever and it's less an adventure then it is a survival. Serah and Noel's is much more "conventional," mysterious boy takes girl on a mission to save the world.
. I remember reading that instead of NPC's giving you the background story, they introduced the Datalog instead. I wanted to read the Datalog, but my boyfriend kept saying "those things are just for the extra curious people, you shouldn't have to read that to know what's going on". In a way, he was wrong.
That's XIII's big problem, world building was in datalog and cutscenes, and cutscenes were a problem because they worked off characters that already knew terms that weren't familiar like cieth, l'cie, and fal'cie. When exposition was done, it was short and easily missable.
I understand that the characters can't interact with NPCs too much, but hell even a Mass Effect dialogue option where you can ask different characters subjects would be more natural. Mass Effect does a whole bunch of exposition drops that are necessary for a first play through, but the writing is good enough and the characters are so well realized that it's not a chore.
XIII-2 story is much better in just that respect, you can gather bits of information just by listening to conversation and sidequests. And the datalog was sort of necessary, but it wasn't a dictionary/encyclopedia that was necessary to understand the fundamental issues of the world. And they were more fun to read in general, because they were more like snipets of diaries than entries in wikipedia.
I really do like XIII's story better, but the execution made it difficult to understand first playthrough (like Fangu said). XIII-2s story was a mess, but it was better told, more freedom, interaction with the world, sidequests that make you get to know some of the issues better (there's a whole Academy versus Cocoon loyalists storyline that's done entirely in sidequests, NPCs convos, and an alt ending), and just better word choice.