Not enough content.
Personally the zones were fantastic and I actually did like the garrisons at least at the start. All the dungeons were fantastic, and the raids were actually good. I could even get behind the story if they actually decided to elaborate on it more, but like my opening statement, there just wasn't enough there.
The expansion itself, even down to concept really wasn't that bad, and nobody can ever really know why it turned out why it did. If people care to remember, there was a pretty giant gap between the end of MoP and the actual announcement of WoD, and the initial announcement of WoD was incredibly bare bones at Blizzcon. I remember being told we would be getting a beta fairly soon after that Blizzcon, only to be waiting for a really long time.
I like to think they weren't completely sold on the expansion itself, but due to how this game operates, and how expansions are launched, they had to throw out something. It's pretty obvious to me there were missteps in the development process and/or certain concepts just absorbed a shitload of development time (I believe it was hinted at that Garrisons in general took a colossal amount of time). I'll never know, but I think part of the problem might have been how they allocated to their resources too. To elaborate, one of the major problems they might have had is looking way too far in the future instead of worrying about the current expansion. Basically looking forward to the next expansion (I remember they had goals of having really short ONE YEAR expansion cycles) without really cementing the one they are about to release.
If those apexis areas were actually fleshed out to be more interesting, and there was something to bridge the expansion story/content between BRF and Tanaan, it likely would have been seen as a pretty good expansion IMO. WoD wasn't bad because of features or the actual content it had, it was just missing the chunk in the middle.
Put it this way. It would be like if Vanilla didn't have BWL/AQ40, TBC didn't have BT/Hyjal, WoTLK didn't have Ulduar/ToGC, Cataclysm didn't have Firelands, MoP didn't have Throne of Thunder, or if Legion didn't have Broken Shore and ToS. To me I didn't really feel WoD deviated too much from nearly ever other expansion, it was just missing the middle part of it, which if you comb through responses in various threads (this one included, and dozens before it) will basically point to that. Even if you hated the story, most people didn't like it because it went from A to Z, without any letters between.