Diablo III isn't an MMO, but in the eyes of many fans, it certainly seems to be aping the style of MMOs, and whether or not that's a good thing depends on your point of view. But according to recent statements from Runic Games co-founder Max Schafer, that's no accident, as many years ago, the game was an MMO.
Schafer states that prior to his departure, the team was essentially aiming at doing for the Diablo franchise what World of Warcraft did to the Warcraft franchise. Changes in upper management prompted Schafer's departure, and his current studio Runic Games is known for its very Diablo-like Torchlight franchise. While it's been a long time since development on a Diablo MMO was in the cards, it seems some of those roots wound up making it to launch after all.