People will always argue about the reasons behind World of Warcraft being so popular. Some argue that Blizzard just spent a ton of money. Some argue that Blizzard has a massive following who will buy anything they put in a box. Some argue that WoW benefited from word of mouth. Some argue that WoW just got lucky.
Wash all of these reasons from the chalk board. Then, sit down, and play World of Warcraft for five minutes. The controls and camera are absolutely smooth. The UI is clean and simple. Tasks and goals are laid out from the start. There is almost no decisions to be made until level 10, when a player receives their first talent point. If a player dies, they run back as a ghost with no experience or item loss. The game NEVER punishes a player until that player is willing and able to avoid the situation.
Fundamentally, WoW is just a pleasure to sit down and play. Take all the reasoning for why WoW is so popular and compare it against this question. If this is truly the reason, then why do people sit down, play, and continue to play?
For WoW, the devil is NOT in the details.