Clearly the annual pass has been a great business move for Blizzard so far, significantly lowering the amount of subs lost last quarter (relatively speaking), as well as assuring that many players with it will most likely purchase MoP. As we know, the deadline for signing up is May 1st, 14 days before D3 release, and then the deal will no longer be available.
That said, do you think they will carry on with doing more annual pass deals in the future? With lots of big titles coming up like SC2:HotS, and probably in a couple years, a D3 expansion, they could provide a lot of incentive to keep people playing WoW while getting expansions for their other Blizzard games for free. There's also no end in terms of what they can do for perks- HotS beta access and free full version upon release being one, unique portraits for SC2, banners and dyes for D3, etc. And of course who wouldn't want a Zergling mount?
Just a thought, I realize it's the "annual" pass not the "perennial" pass but I still think it might be a good move for them to offer new ones every year or two from a business standpoint. I feel like they're doing everything possible to hang on the WoW subs at this point, so I wouldn't be too surprised if they decide to keep dishing out the deals.