Diablo 3 will require players to be online to play. Due to new Battle.net and Auction House features, you cannot play Diablo 3 offline.
That is good and bad thing. For cheating and so on it is good, but you must remember that will be problematic for many users. And playing campaign with online all the time is trouble if you asking me. But dont forget WoW and Blizzard history.
And Diablo III will have a separate auction house that uses actual money. Already stink something here.
It is money,money,money…
More about auction house and money system in Diablo 3 on this LINK.