How do I put it, the game is so huge for anyone who enjoys a good MMORPG there's probably something in it for you.
For many areas, there's a learning curve, in many others there is a big timesink to do stuff.
But as I said before, the game is so huge, you can do a variety of stuff. Soloing, Quest, Group content, Raid Content, Guild Participation, Crafting, Exploring, World PvP, Battlegrounds, Arena, Auctions, Collecting pets, vanity items, Roleplaying. It's got awesome storylines to explore too, and once you've done it as one role, eg: damage. The game can be really different from the perspective of a tank or healer.
There are so many little unnecessary details to the game that make it available to so many people and yet allows everyone to leave their own unique marks on the game. It could just be that you're wearing a complete armour set and you look fully sick. Or you got a rare mount or pet.
But the thing about this game is that it never forces any goals upon you, it never forces you to do a quest to advance, and YOU choose your own goals within the game. Some people play because they love to play the auction house and make money, and apart from levelling up, you never really have to participate in raiding or dungeons to do that.
So you can set your own goals, and with continually growing content, obviously people's goals are constantly changing. Add in the fact that it is multiplayer, people essentially create their own content, their own reasons for playing. People don't have to run guilds, organise raids and so forth, but they do because it gives them a sense of entitlement and privilege that they don't have access to in real life. Let's face it, your average job is pretty lame, boring and unfulfilling, you work for the company. With WoW you can come home and yes, you might be working, but you're fulfilling your own goals.
Not everyone's awesome at sport, it can be a hassle to organise events and find opponents to play against who are your level. In WoW, it's relatively easy to build a team (guild) and verse opponents your level (raiding/pvp) and engage in meaningful teamwork building skills and also social building skills.
At a job interview, I had the balls to tell them that yes I play(ed) WoW. For the regular person a video game is a weird place to be proud of your achievements. But I was able to articulate and explain how I learned to manage people and build great teamwork and interpersonal skills by the social aspect of World of Warcraft. You can't really say that about many other games.
Not to mention the thrill of raids, and the fucking awesome feeling you get when you prevent a wipe and down that pain in the ass boss for the first time.
Then there's the loot, the source of much pain, suffering and joy in WoW.
It's hard to not feel like you've lost something when you fail to get a piece of loot, or someone ninja's it or whatever. You have lost your time, which you pay for. Ultimately though, when you finally get what you want, personalise it with gems and enchants you become attached to your items, your character, and ultimately the game.
Unfortunately for me, WotLK really killed the game. With the introduction of achievements, people stopped just playing for the fun of it, and started playing because they wanted to feel superior by getting achievements. It unnecessarily secluded parts of the community that would normally work together and gave the elitist culture that many top teir guilds and players more ways to feed their superiority. Ultimately, then WoW raiding declined into an era of gearscore and elitism which pretty much killed the fun of the game. It was no longer about trying to help someone else because they're a good friend, a good player, it was about rejecting them because they didn't have the gearscore you felt required to complete the encounter.
It really broke down a lot of the social interaction within the game that had made it so interesting and fun for me particularly in Vanilla WoW and for a long time within Burning Crusade.
Ever since I've stopped playing, I've been trying to overcome a Social Phobia. I think if I didn't have WoW, it would have crippled me a lot sooner, and I would be in a much worse situation than I am now.
WoW's large subscriber numbers also account for people all around the World including Asia and Europe. Many MMORPGs don't participate fully in these markets, particularly in some of the Asian countries and prefer to stick with just Western Countries and also Europe in many cases. This bloats WoW's numbers significantly. If you were to compare subscriber numbers based on Western Country subscriptions you'll find that while WoW is still ahead, there are significant pockets of people invested in other MMORPGs.
Closing Statement/tl;dr -
The game is so big and huge, it doesn't force stuff upon you so there's so much to find awesome in the game. Ultimately it has one of the richest game worlds ever created (and soon to be recreated in Cataclysm), there is so much to do and see and explore that people become invested and obsessed (not addicted) with the game, making it popular.
Whew, long post.