A lot of you will be surprised how easy the content will be. People will clear MC (except Rag) on the day they hit 60. Nef will fall the day BWL will open.
People will know how to perfectly play their characters, how to itemize them, and every single quirk and secret about the boss mechanics. Also you have to keep in mind that "Look yo, that boss makes fire on the floor, and you must walk out of that." was like the craziest mechanic for raiders in those days (and sometimes the only mechanic the boss had), while today this is just one minor mechanic of basically every boss that usually isn't even mentioned when discussing tactics because just everyone knows what to do.
Considering that you have to understand that gear that time was made for people who know nothing, can't play their char, have no plan and can't avoid shit. So the stuff you loot from 60's dungeons will make MC a cake-walk, Dire Maul gear will make BWL easy, and so on, simply because even the worst LFR raider today will still play better than the the average raider in classic.