I don't understand how WoW got so popular. Of the MMO choices I had available to me in 2004 it was the worst one. Dark Age was far superior.. I think it was just the timing. It came out just as people were getting computers that could handle that kind of game, and broadband was becoming ubiquitous.
Not that timing didn't help, because I'm sure it did, but I think there's more to it. I started playing during the beta, and at the time, I was also playing EverQuest and Final Fantasy XI. I've also played EverQuest II, Ultima Online, and D&D Online when it cost money. I cancelled my WoW account earlier this year (just taking a break, I'll reactivate eventually), and occasionally play the free version of D&D Online.
Here's my viewpoint. I played Everquest for a year and a half, and the highest level character I ever had was 12. In FFXI and EQII, it was level 16 in 9 months. I don't remember how far I got in UO, but it wasn't far. None of these games were very "solo friendly", which was my main style of play, because finding other people to play with took serious time and effort.
WoW comes along, and not only is it possible to reach the level cap on your own without serious difficulty (though I wouldn't call it easy, exactly), finding a party seemed a lot less hassle if I needed one. That meant the people that wanted quality raid content had it easier as well. Compared to the other MMOs at the time, it was significantly easier to jump into the action. Combine that with Blizzard's ability to create interesting, balanced content, and you have a recipe for success.