MoP received a ton of hatred before its release. While MoP was played, the only consistent issue the players had against it was the reward structure at release with some decent Valor rewards locked behind rep and that idiotic rep gating system MoP had. Heart of Fear was also poorly received (Garalon sucked).
Legion got hatred for two very related reasons. The AP grind which was a non-issue, it was just the tryhards whining about it non-stop and it did not affect the vast majority of the playerbase; unfortunately the tryhards are VERY LOUD and the legendary lottery which absolutely was a problem.
Neither got significant hatred for their content or their gameplay.
And I am not saying DF is bad. DF is far better than SL or WoD. But so many people are acting as if it is an amazing expansion and I just don't see it.