The healing and to a lesser extent the tanking difficulty is a major issue.
Prior expansions were more of a DPS check.
This time around there's too many degrees of difficulty making M+ generally unfun to participate in, even as an experienced high key runner.
It's very difficult to learn, it's not difficult to execute for a tank. Here's an example:
Ara Kara Boss 1 has an ability called Gossamer Onslaught. It's an AoE that also spawns webs beneath you.
In Heroic you just step out of the webs and the healer deals with the damage.
In M0 you start to notice your health going down and you need to use a defensive.
By M4 the defensive isn't enough anymore, this is when you learn that the ability has a limited range and you can use a movement skill to avoid it entirely.
By M8 the webs start to get unmanageable as the encounter lasts longer and you learn to purposefully place them around the edge of the arena.
I really like that difficulty curve but it's not a difficult boss. Kite from adds then jump out of webs and run around edge of arena.
The problem is tha this stuff isn't usually mentioned in guide videos or the dungeon journal. You don't naturally learn it in lower difficulties as initially you don't even notice the ability doing much damage. You only learn it by dying or almost dying.
If you die you make your group angry. If you talk you make your group angry. Because the timer, because of earning the key. It's unpleasant and people don't develop tactics and share information.
They designed a system where relatively easy dungeons feel like shit to do and hard to learn.
I tried healing and it felt nerfed to the point I didn't bother trying much. I usually heal as my main and healed original MC, BWL and AQ40 as a Druid. It's the first expansion I didn't main my Priest I made in 2006.
They need to remove the timer, remove the keys and buff healers. That would allow them to increase the mechanics difficulty.