The biggest criticism of FFII is definitely the leveling up system. However, once you know how it works, it becomes quite fun to exploit it and then it's up to you to define the difficulty that you want to set.
What really pisses me off are the dungeons with tons of dead ends and room traps with monsters and no treasure chests. That's a jerk move that appears throughout the entire game. Why?!!
I guess it gets more hate than it deserves.