Activision Blizzard held their Q1 2011 financial earnings call with investors and the press yesterday (9 May 2011). During the conference the mega-publisher discussed their continued financial success, the future of the Call of Duty franchise, and the Diablo III beta; among other things.
One of those other things was a discussion about future development of World of WarCraft expansion packs. Blizzard has apparently been dealing with a growing level of WoW subscriber churn in the wake of each expansion release. Churn is a measurement of the number of overall subscribers that leave a service over a given measurement period.

Mike Morhaime
Mike Morhaime, Blizzard president, explained why this was the case. “As our players have become more experienced playing World of Warcraft over many years, they have become much better and much faster at consuming content. So I think with Cataclysm they were able to consume the content faster than with previous expansions, but that’s why we’re working on developing more content.”
“What we have seen so far is that people have been consuming this content very quickly, and so the subscriber levels have decreased [following the release of Cataclysm] faster than in previous expansions,” said Morhaime.
“We need to be faster at delivering content to players. So that’s one of the reasons that we’re looking to decrease the amount of time in-between expansions,” said Morhaime. He was unwilling to share any details on future Wow expansions.
WoW expansions: faster, harder, more << Comments and views
Source: Gamasutra