I'm really shocked at how myopic most people are; they talk about "I pay $60 for a game, and I expect to get all the content."
Well, with World of Warcraft which seems to be the piñata here, you are: any updates to the 'version' you're in come free. It's when they add substantial new content, like Outlands or Northrend, that you have to buy another disk. With Cataclysm being kind of an exception (since it's a reshaping of the original continets and only 5 new zones), you are getting substantially new areas to explore and new critters and bosses to fight.
But that $15 dollars a month... laying aside the fact that if you only played WoW for 2 hours a week, you'd be running about a $2/hr entertainment rate... still better than most of your options out there...
Has anyone really given any thought to what is going on behind the scenes? Each month, Blizzard has to pay hosting costs, maintenance and hardware replacement costs, bandwidth costs, as well as continued support and development, customer service, etc.
That is not to say that Blizzard is operating at a zero-sum, but the point is that I wouldn't be surprised if Blizzard saw less than $5 of the monthly subscription rate as profit, and tbh, that is probably as thin a buffer as I'd be willing to go if I were in charge of WoW, given that things can happen that make that buffer rapidly dry up.