Caius is a pretty great villain but holy shit am I fucking sick of Liam O'Brien.
The Crystarium is tweaked in a neat way; rather than linear paths for each individual role, the characters (and monsters) each get one crystarium path, and each node gives you the choice of which role to level, and gives you a fixed amount of STR, MAG, or HP depending on which role you level. So if you, say, get Thunder at level 10 RAV, you would get Thunder once you'd leveled your tenth RAV node as Serah or Noel. The monsters only have one role each.
This WOULD give you some choice in terms of how to build Serah and Noel, except every other node is a large node, which gives an extra bonus based on which role you use to level it. COM gets +2 STR and RAV gets +2 MAG, and since you can cap all roles at 99 there's literally no reason to not have Noel level COM on every large node possible and have Serah level RAV on them (unless you're some sort of godless communist that hates min-maxing). It's still maybe more interesting than normal leveling...?
Monsters are more interesting, since you customize their skills by feeding them to each other but get ALL the skills (I think; I'm not clear on it yet) and each monster has a cap on the number of skills it can get, so you have to do some planning to make sure Awesome Monster X gets Awesome Skill Y without getting too many of Shit Skills A through M.
EDIT: Also, it's worth noting that I DID play Radiant Historia the whole way through, which at 30 is about the highest praise I can give an RPG, so I don't mean to sound like I thought it was bad. I just thought the combat was kind of dull (the mechanics are great and make the boss fights an absolute treat, but the random battles are WAY too long) and the game had general pacing issues. The characters are, on the other hand, great. I am probably alone in this but I actually especially liked Raynie and Marco, who are mopes and know they are mopes but are also professionals and are loyal to their commander, and the game actually treats them with a lot of dignity when it would be so, so easy to make them comic relief.