10-60 I mixed in dungeons with questing. It wasn't too bad.
My team is 5 druids and full heirloom with the other pieces typically current via a quest reward.
Hit Wrath/Burning Crusade 60-80, wow this stuff actually somewhat hurts! None the less I was able to do every dungeon at least once, and I will say it was sort of fun the first time through because there was a degree of skill involved that wasn't there in the past.

I'm at MoP/Cata and... it's beyond what a "normal" dungeon should be. Currently lvl 88 and have spent most of it questing.
I feel like my combo is strong for PvE and I can handle mechanics appropriately. Less than 6 months ago I was doing Mythic +10 in ilvl 905 gear.., but this. This is at the same tier difficulty, with no real option to change it.
My Bear tank was getting 2 shot by mini bosses.
I was able to complete Temple of the Jade Serpent and Mogushan Palace, but as I've gotten higher in the 80's something weird has happened. Each level it gets increasingly worse, where I just left multiple other places. Questing is faster, and if it gives me this much trouble in a coordinated group I can't even imagine what new players or those without heirlooms have to deal with.

They must notice a downtick in players doing these dungeons, or at the very least if they could track abandoned rate.