Goddamned Bats: They probably won't kill the player characters, but they will make fights drag on longer:
Enemies that cast effects like Daze (you cannot move or attack, but can still defend and use items) and Frozen (you cannot do anything).
Ghost type enemies can make characters in the party drop equipment in fear, requiring you to waste an action putting it back on again.
Scrappy Mechanic: Holding The R2 button to skip attack animations. This is a good idea, but for player characters it is only available from the second and subsequent uses of a power in a battle, and after a while you might just want to skip them every time. It is less predictable for enemies, but probably on a per-enemy basis, so if several enemies use the same attack, you have to sit through the first time animation for every one of them.