FF13 combines the worst parts of FF4, FF10 and FF12 with the good parts of FF10 and FF12.
Later in the game, normal battles are far too long, for too little reward, and just turn battling from being fun into a chore. Why should it take 5 minutes just to kill a standard monster that isn't even a Boss battle? I'd prefer frequent battles that were nice and fast instead of few battles with take for fucking ever to slog through for next to no reward.
Annoyingly restrictive linearity. I know other Final Fantasy games were linear, but at least you could progress through them at your own speed, with long narrow paths with set monster encounters it feels like you're being led down them and thus being taken at the speed the game wants you to move at, instead of how you want to play.
No money from battles. The only way to get extra money in this game is by crossing your fingers and hoping that last 5-minute long battle you fought against that giant (yet non-Boss) monster with give you something you can sell for maybe 150 Gil. You want to save up to buy materials to upgrade your weapon power? Well you'd better hope that the battles you fight actually drop anything at all afterwards, or you've just wasted another five minutes for fuck all.
But it does have a few good points.
Ability progression piece-by-piece, and no fucking shitty random stat increases at level up where the whims of the Dice Gods can fuck you over by rolling low on vital attributes.
Being able to pre-set battle tactics so you can try out different combinations for the most effective approach. Would be more fun if the battles were actually enjoyable by mid-to-late game, but the principle is there.
But the worst thing about this game?
Instant Game Over if the character you're controlling dies. What, both your teammates can cast Raise? So? Too fucking bad. All the enemies decide to attack you at the same time and remove your HP faster than the Medic can restore it? Tough shit, it's Game Over, even if both the other people are still at full HP.
Which idiot decided that was a good idea for the battle mechanics?