Cata is when the game stopped being "old WoW" and started being "new WoW". Almost every part of the game got heavily revamped. Most notably 90% of the old-world zones receiving varying degrees of changes. Plus it's near-universally cited as the worst / 2nd worst expansion.
But it wouldn't surprise me if Blizzard kept the low-effort gravy train rolling and did a Classic Cataclysm anyways. People can enjoy the heavily homogenized classes, pitifully easy leveling, and woeful lack of raid content. I'll happily enjoy TBC and WOTLK servers.