I think WoW's success was a combination of factors, but it didn't hit 11 million overnight, it has built up to that over it's 4year+ lifespan, it started off pretty well though, but it has has more of a "snowball" life than any huge explosion of players.

A combination of Blizzard's past successes with titles such as Starcraft & Diablo1/2 & Warcraft1/2/3 which meant it already had, as a developer, a fanbase of the company & therefore gamer awareness, coupled with an aggressive marketing campaign (something MMO's have historically been weak at) & the cash to back it all up & delivery of a low technical requirement product that also had a lower level of entry needed by way of a simple UI (most MMO had looked almost like flight simulators till WoW with windows & ui pieces all over the shop).

I'd pin it's success on those factors, WoW was a very thorough look at the MMO marketplace that had a deep understanding of "what worked" & what was an annoyance & barrier to entry, system requirements, the game UI, & ease of play being the 3 biggest "game qualities" that sold it to prior non-gamers.

So luck?
No, but repeating that formula is I think a futile exercise, anyone wanting to reach such heady numbers will need a new formula, trying to out-WoW WoW is pointless because WoW was very thorough in what it did by borrowing from all the existing MMO & continuing to do so since, mixing standard MMO fare with pop culture references & will need to build a success out of an initial release, I think Darkfall, for all it's issues, has an interesting release method & something that will be interesting to watch develop.

I think the fantasy MMO space is crowded, I might get proven wrong on that if the in-development fantasy MMO go on to great successes, if I were to predict the "next big thing" I am going to lay odds it will have a far different theme to what went before WoW, maybe sci-fi, maybe horror, maybe steampunk, it would be nice to see a game that could involve several different types of gameplay though, pairing up the character development that hooks many into MMORPG's with other kinds, like puzzle, point 'n click graphic adventure & text based adventures, platformers, first person shooters, racing & fighting games, something that takes all of those together & melds them seamlessly into a world & not in the mini-game way, questing systems have gotten stale becoming shopping lists of kill this deliver that, it needs some expanding into an activity of multiple disciplines.

The next WoW wont be repeating WoW's regurgitating of ideas with fresh clothes on, it will be an innovator seeking to steal WoW's fans by way of it's own merits & word of mouth backed up by a real ad campaign.