If I recall the development of WoD correctly, it was actually quite tumultuous.
The story, from the getgo, shifted a lot. First there was going to be Garrosh assembling a mongrel horde. Then it was going to be on Draenor, and we'd be entering Draenor through a chronal spire, then it was the Iron horde invasion and we'd go through the dark portal, etc. Then bladespire and Karabor were supposed to be the main cities, then it was shifted to the little hubs outside Ashran. I also get the impression that Garrisons became a larger and larger focus of the expansion as time went on.
I think people get the incorrect impression that "not a lot of work" was put into WoD. I get the impression, rather, that a TON of work WAS put into WoD... but that most of it was ultimately in the wrong places.
Developing ALL of the Garrison buildings and all of its various trappings with entirely new and iterative models, as well as designing all of the garrison systems as well? Allowing players to build buildings, invite in other players, recruit and manage followers, etc, etc? ALL of that stuff was brand-spanking-new in WoD, and, like it or hate it, was probably a tremendous amount of work from the art and programming departments. Same with Ashran. It's clear from the structure of the Battleground that a LOT of work went into it. I've always had the suspicion that blizzard intentionally axed Karabor and Bladespire as the capitals to incentivize participation in Ashran.
As I said though, most of this effort was ultimately misplaced. Garrisons weren't very engaging in the end. Ashran only appealed to PvPers, and only the PvPers that... well, basically just wanted to kill things without ever winning anything. Your character's involvement in the story, due to the very bare "go here and do vague things until you get 100% on the bar" nature of the dailies, basically ended once you finished the leveling up quests. And of course, players were stuck on the ground the entire time, due to Pathfinder, making everything a colossal time sink and disincentivizing casual travel. Why, for instance, would you ever go to Arakk? You'd have to run your ass down there and get harangued by NPCs the entire way. Alliance players would never really ever venture to Frostfire, and vis-versa with horde and Shadowmoon. They were simply too far out of the way. Meaning that players that only played alliance or horde were basically out an entire zone.
And while Blizzard was working on all of those,what I would call misplaced, efforts, they were STILL doing a lot of other things. Draenor is in no way a "barren" landscape. It's incredibly detailed. Shattrath city, for example, is very intricately themed, down to even tiny details inside the buildings. Moreover, you had Blizzard undertaking the update of the player models and, again, like them or dislike them, that was an extremely time-intensive process.
But like I said, a lot of that great theming and detail work ultimately went to waste. I'd wager most players never really bothered with Shattrath. After reaching max level, most players would just go sit in their garrisons, and none of the vague, storyless dailies were really incentivizing enough to take the time to go out into the world on your slow-ass ground mount.
So in summary, you have a TON of work going into WoD, plagued by large design shifts, with an ultimate focus of priorities in systems that didn't end up paying off. Meaning that those gaps in time that were necessitated by the front-end workload of the expansion taking so long were very conspicuous as they worked to complete subsequent pieces.