My list -
1) Terrible gating. Do dailies for rep to buy items you've already got better than from 5 man HCs? Poor design. Epics were ok but again, gating that could only be got past by dailies. Capping the amount of rep per day but allowing multiple ways to get it would have been better.
2) Too much to do. I've still not done all the challenge modes, even though I love that shit because it's too difficult for more casual players to organise AND get all the more mandatory stuff out of the way each week. Need to farm 50 coins, get the rep and the valor so not a liability for the raid and now I am free .... annnnnd it's tuesday ....start again. Previous expacs I've managed to get loremaster. I haven't even finished jade forest this time around, too much to do syndrome strikes again.
3) Scenarios. Ok for telling a story, terrible as a repeatable playing experience. The heroics ones are alright though as you need to be awake for them.
4) Ludicrously overtuned normal raids. T14 especially was utterly brutal, and after the near perfect difficulty spread of DS, nuked a lot of guilds and caused a lot of drama. Lets not even mention garalon. What the fuck were they thinking!
5) Use of LFR to replace actual group content. LFR isn't a proper group playing experience, you are hardly ever going to meet anyone doing it. In LFR there is no way to shine as a player and no way to interact with anyone in order to make friends. It's great for seeing the content tho, so GJ there.
6) Flight time isn't a playing experience. Having to fly around the world is an auto run engaged while watching youtube experience. They should knock it off already. Removing mass summon was just retarded. Busywork isn't engagement!
7) Going back to the caaclysm mistake of catering to the super high end of the playerbase. Blizzard, old buddy, it doesn't matter what you put in front of the top tier raiding community, they will sit through it 24/7 and down it in no time anyway. All trying to tune stuff for specifically them does is exclude everyone else.
Theres loads of positives tho