My wife is showing some interest in Diablo III. Great, it is always nice when we play the same game. But then I realized that something has changed: We played Diablo I on one computer using a single copy of the game. I'm pretty sure that at least for the single-player game that was perfectly legal, with games generally having a single computer license, playable by the whole family. That isn't the case any more for Diablo III: You need a Battle.net account to play Diablo III, and sharing that account is forbidden. Blizzard says so themselves (see point 3 there). If me and my wife want both to play, even if it was on a single computer, we will need to buy two copies of the game.
And yeah, I know that I could get around that, because on a single computer Blizzard would never be able to find out we shared the account. But I'm talking about the principle here: Earlier Blizzard single-player games had single computer licenses, now we are down to single user licenses.