I never played it so I cant give genuine insight. But from what I can say, Burning Crusade and Cata and most of the other big expansions changed so much of the environment (for example, big craters being added in at cata, several locations being removed entirely, etc)
Though I say Burning Crusade made large world changes, Cataclysm is the one I'm most referring to, because like literally a tenth or more of the world was changed in a dramatic and irreversible way.
I can see why people would like to go back and enjoy vanilla, honestly.
Just not me, personally.