WoW was very much a matter of the stars being right. It's a good game. (I wouldn't say great but that's just me.) it runs very well on non-gaming PCs, it had a pre-existing fan base from B-net. It catered to both the PvP and the no-PvP crowds. (I'm not sure if it was the first to do this, but it was one of the first.) All these things came together and WoW exploded.
At this point there is no competing with WoW. There is only competing for the number two spot in the market.
Funny, that's what folks said about Everquest...once.
WoW was much more than being lucky at the right time. Quite the opposite, IMHO, and I've never even played the game. WoW has survived and thrived for several key reasons:
1) Continued a legacy from an established and popular franchise over which they had complete control (and had developed)
2) Provided a unique visual look that emphasized detail over style
3) Dramatically reduced the tedium of the leveling process, based on reports of my friends who have played WoW, CoX, EQ, EQ II, AC, AC II, DAoC, GW and others.
4) Offered free, regular content updates with dramatic storyline updates and considerably new material
5) Offered equally compelling PvE and PvP content
6) Has a Development team that doesn't seem to actively resent the player base
If anything, WoW should have been a second-ran after Everquest II. But as you can see
here, even though EQ started with higher numbers, all EQ II did was leech players from EQ. WoW started with fewer subscribers, but ramped up FAST. By late 2005, no other game could even come close to it. And then it kept going.
There have been plenty of games that could have given it a run from for their money, but only WoW managed to grab the crown. They didn't luck into it...they worked very hard to get it.
Dark Age of Camelot, for example, offered realm vs. realm combat over two years before WoW arrived...but it never captured the popular imagination. EQ offered large-scale raids from nearly the beginning. Ultimate Online had PvP play from day one. Nothing in WoW is revolutionary...but the whole game is a well-executed package from beginning to end...and that's what's kept players in the game for years, now.