People can't even begin to agree what "difficulty" means anymore. If you talk about how pulling two mobs in classic meant you risked death, it's "not mechanically hard so not hard", if you die because you had no potion that isn't difficulty because anyone can have a potion ready, if you run out of mana that's not difficulty because you could have waited for 100% mana after every pull, if you pull additional mobs (because mobs are more densely packed) that's not difficulty but you not being careful and should've just approached from a different angle. Running out of resources isn't any result of difficulty (mobs taking 15-25 seconds to die and dealing quite a lot of damage) because you can theoretically make sure to be 100% for every single mob pull.
In short, if you break down how people on forums treat "difficulty" in this game, you realize that absolutely nothing is difficult anywhere. Avoiding a 1 second instant death mechanic isn't hard, you just have to press strafe within .9 seconds, work on your reaction time scrub. Taking 5%+ of your maximum health every hit by a mob as you literally have no stuns or ways to self-heal isn't difficult, just tedious (as if that's less difficult than mobs that barely damage you at all)
So what is difficulty? Is nothing allowed to be regarded as difficult if it's not difficulty in execution? Is that the only thing that can be regarded as difficult in RPGs, and never pure numbers? Your lowbie tank who blocks 12 damage getting crushed for 1/4 of his healthpool isn't difficult because the healer can top him off (but the mana is bleeding out quickly) isn't any sort of difficulty? How can balancing numbers, characters generally not having many tools for escaping or controlling mobs, interrupts etc, mob spells dealing a lot of damage not be regarded as difficulty? I mean if you start arguing this way, no RPG has ever been difficult, because theoretically playing perfectly will produce a smooth experience (albeit very slow and calculated).
And yet people will most likely experience more wipes in a typical Deadmines or SFK run, than they've experienced in any non-mythic dungeon since BFA was released.
Mistakes in classic dungeons mean wipes, in retail it's a minor annoyance with zero consequences.
Leveling or doing dungeons on Live feels like you are stuck in godmode, and for all intents and purposes you are. You have to play purposefully awful in order to die on Live.
In contrast, no one has proper AoE in DM, you can run with a level 19 paladin who has no heal other than a pretty weak 3 sec cast Holy Light, and a 1h LoH. Mobs have fleeing-mechanics and barely any in the group can slow or stun them.
People can repeat tedious != difficult and similar, they will still experience far more deaths in Classic than they've done in retail for years. Then it doesn't matter that it isn't strictly speaking mechanically hard, that high level mob with a 20 yard aggro range is still going to whoop your ass, and throw a 8 second Net effect on you for good measure.
Well, what I can at least be certain of, is that the tone will change quite drastically when all those who hasn't experienced it will realize just how much of a beating they're taking from an equal-level mob. It's fascinating how much the game dynamic changes when you lose godmode.