TBC shares a lot in common with retail. You can still play through outland and do timewalking stuff. You can't replay Vanilla WoW on retail. Vanilla is very different from the expansions. Especially considering players would be transferring their max level characters. The design decisions that plague Battle for Azeroth were born in The Burning Crusade.
Bland and boring Outland leveling that can be finished in a week.
Faction imbalance issues that were prevalent when Blood Elves joined the Horde.
Horde racials even more op than Alliance. If you think population imbalance is bad on classic pvp servers, well imagine tbc.
TBC sort of ruined Warcraft lore. It was destroyed in The Burning Crusade.
Being stuck on a 7 zone continent, with zones that were often empty like Blade's Edge Mountains.
Flying mounts killed off World PvP and shrunk the 7 zone continent to an even smaller size.
Remeber how grindy the attunements were just to do a raid.
Uncreative TBC dungeons. Wings and hallways full of copy paste.
Imbalanced Arena. Imagine nowadays with all the min maxing. Players will just play meta comps.
Remember how disruptive Resilience was to World PvP. It rendered many dps specs unviable.
Gear vendors. A RPG where you buy all your gear off of a merchant in town.
Daily quests inflating the economy. Repetitive and timegated.
Remember how everyone on both factions flew in circles around Shattrah all day.
Remember how awful and boring Shattrah was as a capital city.
Remember what a chore raiding Black Temple for 1 year was or how mundane Hyjal was with the trash pulls.
I just don't see the appeal for TBC. Nostalgia is a real thing you know.