Ex-Blizzard employee Mike Huang shared his thoughts at his personal blog after the announcement of Diablo III -- which took place at the Blizzard Worldwide Invitationals on June 27th. He slips some details of the early concept work done by Blizzard North prior to their departure on 2003.
The Barbarian class was not planned at that point because it was considered a broken class. All classes in Diablo III were originally planned to be different from the roster in Diablo II to create new skills/talent trees from scratch rather than redo the old classes in 3D.