A lot of people are so hung up over “omg we’re so much better now” and “easy vs difficult”. Even when Vanilla was brand new, it was a straight forward game. Those of us that knew how to play, knew exactly what to do. All that stuff regarding BiS and talent builds, etc were all there if you knew where to look.
And you are correct, all Vanilla content will be cleared day 1 - even Naxx. But the point being is this. We’re on 1.12 and a retail client - trivializing early content. MC, BWL, and even AQ40 are far easier.
There other thing not factored in is that when actually progressing through patches, it made Vanilla much more of a time investment. Want to clear MC as of 1.0? Make those scholo and strath runs with more trash and faster respawns. Spend 2-3 hours in there where one wipe = end of the run. Do less damage with garbage gear and talents. More time, impacted BiS as well as snowballing into guild dynamics. This idea of “we’ll just run MC for Thunderfury” - sorry not going to fly. When AQ40 first opened, MC was still taking 3 hours or so before it was dropped from our raiding rotation.
Why was it dropped? Time. Good luck convincing 39 other people to waste 3 hours of their time farming for a weapon for one person, when they want to progress.
By the time Naxx hit, most raiding guilds were running on fumes. We were tired, jaded, and burned out. We also hit each tier not completely decked out in the best gear. We had to gradually gear up.
Time taken was everything. No, private servers, or Classic, or any version running on a static patch without any sort of a re-tune is no where near the overall experience. And it is has ZERO to do with easy vs hard. It seems like people are so desperate to prove themselves - having oats to sow with the current game.