I think you've failed yourself as a gamer if you are trying to breakdown the amount of time you played and the money you spent in some kind of "equation of just how much fun per cent I've had". That shit is just sad.
Diablo 3 failed me because not once did it give me that feeling of loot lust.
Not once did I feel connected or responsible for my characters growth, I was more like some passenger riding along a monorail.
Not once did I feel compelled by the story or any of it's characters, in fact I had to actively work towards pushing those things out of my mind.
Not once did I feel like I was playing a diablo game.
I could go on, but many people have articulated this point better then me.