For me, it's the heavy focus on garrisons and the "raid or GTFO" thing that WoD is doing. Garrison chores got boring less than a month after I hit 100 on my main, and I "retired" from raiding at the end of Cataclysm. WoD doesn't offer much else that I care to do.
To elaborate a little more on "heavy focus on garrisons" here, I'll say that I don't like what garrisons did to professions (particularly what the herb garden and mine did to gathering professions). And while garrison missions seemed cool at first, I agree with many that they feel more like a Facebook game than something that should be in an MMORPG. Honestly, I think I want to step down from the "commander" position and go back to being an adventurer. I'm fine with letting some NPC be the commander who sends me on missions.
Also I preferred the original plan of having faction cities on Draenor, instead of people just hanging out in their garrisons all the time.
I won't say I _hated_ WoD, because it had its good points and introduced a lot of good things to the game (leveling was fun, Draenor was beautiful, all the bag optimization stuff, new models, etc), but it's very easily my least favorite expansion in terms of actual content. I started playing in 2005, and WoD is the first time I've been unsubscribed longer than active in an expansion. Even Cataclysm at least managed to keep me playing after I hit the level cap.