The accumulation of all the bad story choices they have been making since Cata because they were too scared to just kill of Sylvanas and end the Horde Vs Alliance subplot in MoP.
Also it is the Expansion at least to me with way too many barrier to entry making having alts, being a returning player, or simply trying to change your spec a gold and time investment. Torghast being such a chore while having to be done on every character until it eventually let you trade the resources, how annoying farming Cosmic flux for any legendary you wanted to make while also not being account bound until the end and it needing to be used for tier set creation as well, the sheer amount of gold Legendary armor on the AH is depending on server. The conduit energy…
The zones being split meaning you had to flightmaster through your current zone to oribos then to the next zone each time unlike previous expacs where some zones had teleportations or you could just mount through/fly directly there.
WoD sucked but that had to do with it being another orc expac that felt like filler and it abruptly ending on such a stupid note. SL was just overly inconvenient and honestly I would of preferred if it abruptly ended in Sanctum of Domination.