Well, I mean the game is basically one long dungeon. To be honest, I feel that having to conserve items and worry about MP and HP for fear of a game over is pretty much a dated RPG mechanic. It's frustrating and slows everything down. In FF XIII the challenge lies in how the battles are fought and what paradigms and character-building routes you've taken.
Having to worry about constantly refilling an MP gauge in a battle system as automated and fast-paced as FFXIII would have been mind-numbingly horrible and you know it, especially since "magic" in FFXIII is basically just another variant on the physical attack. It's not really a special thing, it's just a different type of attack.
The Crystarium is...um...haha...I dunno, having to manually level up every aspect of a character's attributes is actually really irritating, especially once you start getting into the higher tiers. In the end, you'll just end up wanting to be sure that your Crystarium is capped for whatever chapter you're on before you end it and have a lot reserved for starting out the next chapter, otherwise you'll find yourself in a world of hurt later.
As for linearity, again I gotta say the clamor for open-endedness is really overrated. I liked how "focused" the game was on the story, and cutting out a lot of the conventional RPG cliches like big overworlds that you get lost in, or running into weird side-quests that totally derail any sense of tension or urgency.
Actually, I thought FFXIII is weakest when it stops being overtly linear, around the second act of the game; and you can tell the developers didn't really like it either, as the sidequests are really meant to be done after the main quest is completed...take note.
I'd have liked a little more variety in terms of gameplay, like more mini-games and action sequences like flying around a ship or something, but honestly the lack of filler, it doesn't bother me.
The main thing I don't like about FF XIII is how much power-leveling is necessary for the game to not be ass-numbingly hard. Finding mobs that respawn quickly, can be ambushed automatically or very easily, and killed quickly is vital, there's like a handful of these in the game and are almost required to be taken advantage of to save time. The end-game also bugs me, it ultimately just becomes about killing big dinosaurs.
Anyway, it seems like your camp won as Final Fantasy XIII-2 has towns, and player-driven plot points with multiple endings. And also seems to be largely about Serah and some spiky haired punk that wasn't ever in the first game? Okay...honestly these -2 Final Fantasy games seem really ill-conceived.
Also, once you've beaten the game, check out this video, it explains the mythology of the game pretty well.The Mythos of Fabula Nova Cyrstallis