And it's not just WoW - I'd go so far as to say it's all MMORPGs.
Kids these days... just not into what they were in the past. The notion of "working for your fun" is just unreal to them.
For multiplayer, they want games like League of Legends, Call of Duty, Halo - games they can pick up easy, and put down easy, with minimal investment.
I think WoW's problem right now is generational - We, the MMO gamer generation, are getting old. We're graduating, getting married, having kids, kids growing up, etc.
And there's no one replacing us. As we age and move on, the next generation just doesn't want to grind. Why pay $15/month on top of the standard game cost when you can play with your friends for just the price of the disc?
Why go through 36+ hours of mind numbing leveling, with no challenge, no excitement, when you can hop into CoD multiplayer on day 1?
While other genres are expanding to try to reach new people and markets, the MMO market is shrinking. Fewer and fewer people are willing to put forth the hours, dedication, and coordination required to "have fun."
My take on WoW's decline - it's not just WoW. It's all MMOs. And it's no one's fault.