In the open Sunday thread the question was asked: "Is WoW's success based on its player base or its quality? There are other games that are very good, even some excellent, so why is WoW this big?"
I don't think there is an easy answer to this. That starts with the problem of how you define "quality". Most people are completely unable to separate gameplay design from quality of execution. Thus if they are playing a game in which the gameplay is fun for them, they will say it is an excellent game of high quality, even if there are obvious flaws in the execution, like lag, bugs, and a bad user interface.
I would say that yes, World of Warcraft has an excellent quality of execution. It does what it sets out to do very well. But of course that doesn't help you if you either don't like the guided approach to gameplay, or if you did like WoW and burned out after having played it for hundreds or even thousands of hours.
The second part of my answer is that World of Warcraft is big because of its accessibility. Some people laughed when Blizzard revealed that only 30% of players who are playing the free trial for WoW make it past level 10, but several industry veterans stated that this is far higher than the retention rate of other MMORPGs. How many 6-year old PC games do you know which still regularly hit the top ten of the PC games' sales charts? Apart from the problem in China, subscription numbers for World of Warcraft have held up steadily. Not because nobody ever quits WoW, but because there is still a steady stream of new players joining this game which balances the exodus of people who got bored with WoW. This is only possible because World of Warcraft is relatively newbie-friendly. Of course it is easy for the elitist jerks to paint accessibility as "WoW is dumb" or it "caters to the lowest common denominator". But actually the challenge level of the end game is completely independent from the ease of accessibility for new players. As some readers pointed out, very few players actually beat the Lich King on 25-man heroic.
Does the success of World of Warcraft have to do with its player base? You hear a lot of conflicting statements on that. On the one hand people claim that WoW is a completely anti-social game, in which everybody just soloes, and "pickup group" is a derogatory term. On the other hand some claim that people only play WoW because all of their friends are playing. Obviously both statements can't be true at the same time. I do think World of Warcraft is far more social than some people think, and the ability to play solo or in pickup groups does not actually destroy the social coherence of the game. That isn't to say that I wouldn't like WoW to introduce a bigger group XP bonus to get more people to play together, or that I'm not looking forward to the new guild functions in Cataclysm. But at the same time I'd like WoW to introduce pickup raids. Because I don't think you can create a social network if you only play with the handful of friends you already have, meeting new people, and making new friends is an important part too.
The fundamental problem of the discussion of why World of Warcraft is so successful is that everybody is aware that any statement of "WoW is successful because of its high quality" can be turned around and interpreted as "this other game is not so successful because it is of lesser quality". While I do believe that this is very often the case, very few players of other games would be willing to admit that. It's like if your favorite sports team loses horribly against another team, you'll never admit that your team just plain sucks, but rather invent stories of the referee not having been impartial, or some external condition favoring the other team. Thus in the discussion of World of Warcraft you will still find completely spurious arguments of how WoW is only so big because of marketing, or because it makes its players "addicted", or other such nonsense. Not because people actually believe that, but because they are defending their own favorite game with pseudo-religious favor, and have to explain away the success of World of Warcraft.