WoD brought back a lot of players because of the lore and concepts, the leveling experience was pretty good and the Raids were fun, but pretty much everything else was missing, so once you did the main campaign quest and started raiding there was nothing to really do. They also cut a raid tier, and one of the big patches literally just gave us a selfie cam, which pissed people off even more. They also changed the ending last second so that we all of a sudden just became friends with Gromm for no reason.
Oh, and the RNG was retarded. You could have the same opening and pull on the same boss and see an over 10% damage difference based on how lucky you got, and in Mythic mode this could really fuck up an attempt if too many players hit the low end of their luck. There was also the problem that once a raid tier came out, all other tiers became meaningless, but that existed for a while, however because High Maul and Black Rock were the same tier in theory it was even more pronounced as Black Rock made High Maul pointless. Though I might be salty because we were just about to start on Mythic Imperator when BRF came out, so I ended up missing out on that fight.
Warlords was the biggest wasted potential in WoW in my time playing. On paper, it was an interesting concept: an opportunity to experience Draenor before its destruction, to explore its lands in a (relatively) unsullied state, to meet characters that hadn't really been seen since the early RTS games. Like you said, those concepts brought a lot of players back because their prospects were intriguing, and many wanted to see how it would all play out.
Instead, it fell victim to Blizzard's endless quest for more money as they tried to shift to a yearly expansion model part of the way through development. Multiple chunks of content were cut out entirely, like the ogre continent that was supposed to be to the southwest (and is still visible in the map), the Shattrath raid, and the island of Farahlon, all because they couldn't develop all of it and still have the next expansion out in a year. The players' negative response to the lack of content likely played a factor as well, with the devs deciding to cut their losses, end with only one major patch (I'd honestly forgotten about the insult that was patch 6.1, the fucking selfie cam patch), and rush to Legion as fast as possible.
And despite all the cut content, they
still missed their one-year target by nine months, with patch 6.2 lasting for
fourteen months. Fourteen months of only a single raid being worthwhile content, and pretty much jackshit else to do.
At least they learned not to try and pump out expansions that quickly, and Legion did have a decent content release schedule. But it still hurts knowing what we could have had if Blizzard hadn't been so retarded. Then again, that's pretty much WoW as a whole, isn't it?