My wife and I both play, and have for years. My tendency is to play too much, get all the gear, and achievements, get frustrated by something stupid and quit for a while -- only to come back a few months later. My wife plays casually, *loves* the graphics, and all the little frills (pets, mounts, etc.), and actually *reads* quest text and follows the story. I tend to think she's doing it right, and I'm (usually) doing it wrong.
We'll be getting MoP and likely enjoying it enough to pay the fees.