With Diablo III, the developers have started development differently than the Starcraft II team has done with the RTS game. In Starcraft II, the game was created with Multiplayer in mind, first. Then the Single Player got started afterwards. As of the Blizzard Worldwide Invitationals in Paris, we were told there that Starcraft 2 Single Player was 1/3 done.
Diablo III, by the other hand, has been built from the ground up as a Single Player game at the core, but co-operative gameplay is enabled by default in the single player version.
Jay closes the interview hinting that there are pretty awesome features in Diablo 3 that haven't been announced yet. Read the full interview.
Jay: Well one of the things we’ve carried over is the way the general co-op features work,. When you’re playing the game you can have another player jump into your game any time so it’s very dynamic - the game adjusts automatically to the number of players who are there. One of the ways we’ve really improved that is what I like to say is our prime directive is “thou shall do no harm to the co-operative game” - something that Diablo 2 didn’t really follow.
They liked the idea that players can compete against one another while being co-operative. But I think - while a noble experiment - it mostly proved to make players not like each other and not want to play together. The average game size on battle.net in Diablo 2 games is 1.2 players - which basically means almost everyone is playing games by themselves. One of the main reasons is “why would you want to play with someone else?” They can go hostile at any time and kill you, mostly in an exploitive way and it’s not like it’s a fair fight - you’d probably be dead before you even noticed they went hostile; there’s a good chance they’ll steal all your loot so you won’t get anything good and it just makes the game a little bit harder so why would you want to play that way?