I'm playing through FFII on my ipod touch and I'm getting sick of looking at the same 2D sprites all the time.
They aren't bad, poorly designed, or ugly to look at. The problem is simply fatigue. I've been looking at the same character sprites, spell graphics, attack animations, and re-skinned enemies for over 15 total hours of play. The problem is most likely compounded by the fact that I immediately went from playing FFI to FFII, both of which have been redesigned in the same style.
For some reason the 2D sprite art style really brought this phenomenon to my attention, but it's definitely not limited to 2D games. How many games have you stopped playing, not because of the gameplay, but because you were sick of seeing the same graphics over and over? I know it's happened to me a bunch.
Gameplay is definitely the centerpiece of a game, but don't underestimate the impact of the art style and graphics. Look at games like Limbo and Bastion to see the truth in that. They're games with decent, but not wonderful, gameplay but they're unique selling point is their art style.
I really appreciate it when developers take the time to introduce variety and creativity in their art style. It can keep me playing a game just to see what's around the next corner. And that sense of wonder and excitement goes a long way.