Diablo isn't a child's game and doesn't need to cater schoolboys who get an hour or less a day to play. It's always been the hardcore game in blizzard's arsenal and I imagine D3 will be no different.Starcraft?
Seeing as it's not a subscription based model, there needs to be an extensive amount of gameplay packaged in the initial release. Otherwise the game will be reviewed poorly prior to enough sales being made. It's not a game that's going to get content updates every few months like an MMO. I imagine it will be shocking how hard it is to hit cap.