Diablo was going to be a turn-based RPG, but after David Brevik successfully pitched the game to Blizzard, Blizzard had a couple of changes that they insisted on. The first was that they wanted multiplayer, but the big one was making the combat real-time. Brevik did not want to do that, as he felt it would take too long to develop the new combat system. Apparently, he held the decision to a vote at the studio, and real-time combat won out.
To think Diablo would have been VERY different if Brevik had decided that he was the boss and he didn't have to listen to the vote, or even do the vote in the first place. But he did, and thankfully, respected what everyone at the studio voted on. It ended up not taking him very long to create the real-time combat system.