WoW's biggest issue is the massive swings they take the developers take now.
"Mists has too many dailies, let's launch the next expansion with NONE!"
Mists was whatever, but it gave reasons to actually keep playing the damn thing. The new zones content wise were neat, and the raids were good. WoD doesn't have any of this going for it. There was 1 content patch, and from most accounts the zone added was shit.
Garrisons may be part of the problem, but people are putting way too much on it being the problem. The same expansion that removed daily quests also removed valor points, so there's no reason to do daily dungeons either. You don't give people anything to do, people stop logging in. People stop logging in, their friends get lonely/bored and do the same, repeat til they're losing people in droves(which is what's happened here).
I mean, you don't suddenly get that many people again, and lose that large of a % of them without something being very very wrong. Warlords being only one content patch reeks of a massive panic button being pressed somewhere.