For me, one of the biggest deathblows for WoW was when dungeon progression was put on a timer. I can't think of anyone that enjoys timers, and yet, the most theoretically accessible form of progress in the game is hidden behind just that.
What if I just want to enjoy some challenging content with a small group of friends, sink a couple of hours into a long, hard dungeon, chill, talk shit, have a few beers? Oh right, can't have that, the last time Blizz tried something like that was revamped Kara, and to be honest, it didn't quite work, in my opinion - at least I didn't get the same feels from running it with friends as I did running Strat/Scholo/BRS/BRD, or the harder dungeons in BC/WotLK era.
Timers were cool when they were used for leaderboards in MoP, when the biggest thing you could get out of it was a mount or cosmetics. Flex your massive peen? Check. Challenging content? Check. Timer? Check. Oh yeah and gear normalization too, so you couldn't push timers by just having more stats on your loot.
Making it core gameplay essential to progression, on the other hand, was dumb as fuck and drove a giant stake made of shit into the already suffering mode. Yeah there's M0, but who the hell runs M0? Why would you want to waste the time of your friends like that? Oh yeah and you can't waste time of randoms because you can't LFG it, tee hee.