Mixed bag.
MC was always going to be a poor metric. It was never an especially hard raid to begin with, so much as it took some very specific gear people weren’t looking out for early on (itself bypassed on these kills by a world buff that only works well if you quickly oneshot the place and was typically used for farm runs instead of progression – and even that wasn’t enough back in the day), nobody knew how to herd 40 cats, Rag wasn’t summonable for the first month at all (not just the rep, but you had to farm Executus for some 4 quest drops that don’t exist anymore), even once you summoned him his script was bugged to be unkillable for quite a while after (submerge phase never ended), attempts were limited because his summon only lasted so long, and we’re running talent trees and drop tables from a vanilla patch that post-dates the raid by two years (those greens they walked in with weren’t arbitrary, they were specific secondaries that were fantastically rare before AQ or so and more powerful than most of MC’s drop table). Attrition fights didn’t get especially demanding until BWL, and mechanics didn’t get at all complicated until AQ. MC in general was an anomaly in a lot of ways, even before you factor in “no-name” guilds who happen to have been practicing this exact speedrun together for the better part of 5 years on private servers.
For the record, don’t expect BWL to be last much longer. The main gates there were that Chromaggus was broken, and farming Onyxia cloaks for the whole raid to survive Nef’s nukes – something that took several months for a whole raid, but BWL is two whole release phases away. We’re going to have to wait until AQ and Naxx to really see much that approaches even the scale of apples and oranges.
And we’re never going to get apples to apples. Comparing classic raids in general to retail Mythic is going to be a mistake. Vanilla raid end bosses were attrition and gear checks tuned around farming the place for gear for several weeks, and often artificially gated by bugs. Current Mythic bosses and DPS rotations are mechanically far more complex, but also designed to be killed within a couple weeks of release while the world first race hype is still fresh. They’re literally not tuned around testing – the numbers are blindly scaled-up spitballs by people who aren’t actually capable of killing them in any level of gear (and don’t even appear to be very proficient at understanding the mechanics and itemization they created), then every couple days they nerf the current brick wall until someone makes the kill.
Now, as far as everything else? The difficulty seen by the average non-raider is utterly beyond retail.
Leveling, classic is harder, full stop. The resource starvation in early leveling is completely unheard of in retail, and even once your spellbook is full and your mana pool is beefier the overworld still demands proficient field control just to stay alive. Plus, this is the first time a lot of retail folks have ever seen a mob higher level than themselves, which carries sharply increasing penalties to hit chance, damage done, damage taken, and even aggro radius. Very few elites are soloable by very few classes, and it takes a lot of kiting to do it.
5-mans? Classic is harder again, by a longshot. Tank threat in classic is a shadow of what people are used to (and can miss), and failing to focus fire and CC properly means someone – potentially the healer – is getting mauled. Mob damage output in retail is completely trivial until mid-level Mythic keys, and most of it is preventable casts. Classic mobs hit like trucks just with their autoattacks. Also almost anything humanoid flees at low health, much faster than the few retail mobs that do the same. You have to get pretty high into M+ to see anything anywhere near as threatening as LBRS or Strat.