Blizzard mentioned development of a MMORG in the late 1990's and started teasing in the early 2000's, and really ramped it up prior to launch.
And when it landed, holy hell: it devastated worker's and student's sleep schedules for years.
Plus it lived up to the hype: the world was huge and immersive, and they had tuned raiding & PvP based on the experience with co-operative Battle.net.