According to Street, “We find that expansions are what bring players back to World of Warcraft. Really good patches will keep them, but they aren’t as good at bringing players back to the game.
“We really want to get to a cadence where we can release expansions more quickly. Once a year I think would be a good rate. I think the best thing we can do for new players is to keep coming out with regular content updates.”
The difference between major updates and expansions are minimal at best, it seems, and that putting updates in a box draws subscribers instead of merely keeping the current ones happy.
The newest expansion, Warlords of Draenor, was announced at BlizzCon 2013 on the 8th November. No set release date has been given.