At least a million gone. A friend bought me a month of WoW time for an early birthday gift (for some reason; I appreciate the gesture, at least?) so I've been leveling a Death Knight and Wyrmrest Accord feels empty. So does Moon Guard. The two highest-population RP servers (which regularly filled up and needed a queue after Warlords' launch issues were cleared up) feel like ghost towns outside primetime.
I suspect heads are going to end up rolling before the dust from WoD settles.