Originally Posted by
.Nensec
In truth it isn't. Anymore. It used to be much greater than the competition because it managed to put together a lot of good things when the MMO scene was relatively new. They started at the right time, when just about everyone was getting broadband internet.
The game required very low system requirements ( I remember playing it on a single core amd sempron, on board graphics and 512mb ram. Fuck ye. ) so anyone could play it and they largely still keep this formula to this day; you can practically play this game on a toaster if you want to.
Nowadays the game isn't much better than the competition. RIFT, SWtOR, TERA, GW2, etc.. all have their ways of being better than WoW but in the end WoW has history on it's side. Many players are reluctant ( myself included, damn it all ) to start 'anew'. Dropping all the accomplishments gathered over the year and effectively turning them into dust. This is what keeps many players playing too; they don't want their accomplishments to be futile. Ther's a reason I still walk around with "Death's Demise" to this day.
Sure the game is innovating with things, adding in a bunch of technology to push the MMO scene forward. But many have already been done in beforementioned games. They just never reached the popularity to remain as alive and vivid as WoW does. The game has, what, 9 million players left now? That's still about -~8- times as many as all the other games -combined-.
What the other games lack is a buffer where they can fuck about in. WoW can do a lot of things that don't work out 100% and get away with it, if a new game does it then it generally quickly falls apart and the game turns F2P to save it's own ass from certain downfall.
There are many more reasons, such as a very rich lore spanning from '90s to now with so many story twists and spin off plots somewhere that we haven't even seen yet ( Turalyon anyone? ) that can almost consist of an entire expansion alone if you elongate the story a bit and invent some new enemies to fill the gaps ( Hello, Pandaland ).