Well, I would guess its the artistry they put into it. The music is orchestrated. Not like those cheesy medieval mandolin sounding games. Even though the graphics is not exemplary and is sort of dated, the setting and the creativity they put into it makes it immersive and novel. If you have good speakers its even better.
The world is flexible and they make up the stories and environment. They dont have to stick to copyrighted lore like LotR. Its often not too serious and has some humor in it. And there is variety as well to suit different tastes.
But most importantly, they put a lot of effort into balancing the game and economy. Unlike other games which often has some power build for a long time, they nerf things left and right. And they even change things that arent messed up just to give the whole game a different dynamic.
The technology is pretty good too as it often seems fairly smooth compared to other games. Its easy to play, yet has elements for the hard-core. It caters to both casual and the enthusiast. People make all sorts of interfaces for it.
Perhaps another thing they do that is important to some people is that they moderate fairly strictly. You can curse someone out just a little and be reported and banned for a few hours to life after a few too many warnings.
And Blizzard has a reputation and award winning tracks.
I dont play it anymore, but after experiencing it, I would definitely be able to see why its so successful. All I have to say is that if you havent played it, this is the game to try out.
And you can join this thread in this forum on WoW that has gone on for like 461 posts! LOL