changing party leader was something you could do in XIII.... you can just optimize your paradigms based on who you had on point.
Monster collecting didn't necessarily add much to paradigms that wasn't already present in XIII.
Monster collecting definitely added and makes significant changes to the party just by having to be able to change that third party member. Just like how having a monster in your party in X-2 also mixed things up. Changing party leader was not something you could do in XIII, at least not in mid-battle. Optimizing your paradigms also came in pretty late in the game.
I disagree. There's several approaches you can take to bosses in XIII and still do well, I played XIII under NCU conditions and still had no issues getting 5 stars on storyline bosses and most quest bosses.
Also momentum is actually simpler in X-2. Momentum in X-2 is just reflected in chain combos which are pretty easy to stack.
In XIII, you have more factors going into keeping momentum in battle because enemy stagger isn't as easy. (ATB refresh, Eidolon cancel, etc.)
I'm sure with bosses they have to be a little more versatile.
X-2 is more about timing, having to attack the same monster at the right time will cause a chain. The problem with XIII it doesn't exactly "spell it out". This is one of the minor identity crisis of XIII as it wants you to focus on paradigms, but in order to get the 5 star process,
with XIII it also focus on momentum on beating the game, and i'm not talking about just enemies, but how the game is played. heal enough, then hit hard enough, then heal enough. its this "flow" that you run. And other FF games have their own flow, but more based on specific tactics. You can still win in auto-mode, you just wont always get 5 stars. For a specific boss of course, Barthandelus, is a special exception.