From the viewpoint of a casual mythic progression raider (i.e. pushing mythic but at no incredible clip, I'm 12/13 Heroic right now) and a guy who liked to PvP on the side with friends, here is what I believe went wrong:
Garrisons. A facebook desktop game made its way into an MMORPG, rife with RNG, gating, and other infuriating annoyances. Instanced areas that isolate players more than ever before with rewards that actively discourage players from leaving.
Lack of end-game content. As a PvE raider, I can speak on this quite well: the raids are great, but that is all. Once you reach maximum level, there is nothing to do but raid, and that is unacceptable. This is evidenced by not feeling the need to log in other than to raid. While I do not mind this, it is surely not the way things should have gone for the majority of players that do not raid on the level that I do.
PvP imbalance and the approach to PvP in WoD. Essentially, Blizzard tried to shoehorn people into doing the PvP content that they wanted the players to do, which was met with overwhelming outrcry. Ashran, aka Trashran, aka Assran, was forced upon PvP players for no less than eight months - a broken excuse for a zone that was advertised as the evolution of Wintergrasp and Tol Barad but ended up being a design nightmare because of a distinct lack of focus for what players were supposed to strive for within the zone. In addition, at this moment in time, faction imbalance is the worst I, as a nine year veteran of this game, have ever seen. Alliance win nearly every battleground seventy percent of the time. While people are reluctant to admit it, or are too ignorant to see it, racial imbalance has led up to this eventuality because of the trickle down effect of top players faction changing, then the step below, continuing ad infinium. They're starting to turn things around, but it's too little too late. Did I mention that no new battleground was added? Long story short, I have felt no desire to PvP this expansion because of the sham it has become. I eagerly await the new systems being put in place for Legion - perhaps PvE and PvP imbalance will at long last be rectified.
Value for money. For ten dollars more for the expansion, we as a playerbase received effectively a quarter of the content total, most of which that was promised being cut. What remains are horribly implemented currency and reputation grinds and raiding. Dungeons are irrelevant to a degree I didn't think was possible.
WoD was slated to be one of the best expansions from a gameplay standpoint when it was announced. But, after experiencing it first hand...it is one of the worst. I remain quite hopeful for Legion, as one of the main selling points, class identity, is something I would greatly appreciate as a Death Knight.