There's no arguing that World of Warcraft is one of, if not the most, successful MMORPGs out there to date. With its subscription base in the millions, it has received a great deal of criticism and praise by both reviewers and players. Having played various MMORPGs for the past 6+ years, I've taken a look into what makes WoW so successful while others have failed, and have come up with the following reasons.
The number one reason that I have found for WoW's success it its foundation. Yes, that's the title for my blog; fitting first post then, huh? What I mean by this is the Warcraft universe isn't based on the MMO title alone. It has a rich background previous RTS titles that have taken the liberty of providing the setting, lore, races, classes, enemies, etc... Not only that, but those titles also created a dedicated following, many of which migrated to WoW at its release. To my recollection, I have not come across another MMO with this track record.
The second reason why I believe WoW success is so pervasive is due to Blizzard Entertainment's philosophy. Love it or hate it, the policy of releasing "when it's done" has done wonders for their game(s). I realize that being able to take their time on games to make them polished before release is a luxury not every company can afford, but part of this is from Blizzard's monetary success with their non-MMO titles.
With these thoughts in mind, why don't other developers follow suit and come up with a series of non-MMO games to build up a potential support base? Sure, you won't be able to jump on the MMO cash cow right away, but it would give the chance to see if ideas would work and the potential for user feedback on game aspects in regards to lore and setting. Plus, if an idea was successful, the companies could start gathering a following before releasing le grande MMO (if that's what they decide to do in the end.)
I don't see WoW going away any time soon. I do foresee a lot of other MMOs coming and going. Granted, there are MMO titles out there that have been successful without the history of predecessors in the same universe, but look at the numbers and there's still a big gap between subscriptions in games.
Will companies start following Blizzard's patterns of success before the advent of WoW and follow suit? I can't say, but I think the greater potential for success lies along that route.