Warlords of Draenor
WoD lacked a lot of content. There was barely any outdoor content or incentive to leave the garrison. A lot of effort was put in to events that happened inside the garrison that were ultimately a waste of time. The mini companion mobile/facebook game was fun for a tiny while, but quickly ran out of steam. Especially after Alts started getting in the picture. It was just an expansion without content. A lot of it I think is the fault of mistakes made in Mists. It kept the developers afraid of the old style endgame content and they maybe tried too hard to innovate new things.
Raids of course were fine, as they've always been aside a few exceptions, but it wasn't enough. Dungeons were fine, thought they could have been harder. I loved challenge modes as well. It's sad that they're gone now.
Having played from Vanilla, WoD has been the worst so far. There were a lot of cool minor innovations done in WoD that made the game better. It was the overall presentation that didn't seem to have a direction.