Currencies and systems. Systems and currencies.
What is most appealing to me about TBC is getting away from all the convoluted systems and currencies. The game was a lot easier to grasp back then and it didn't feel like you had a gazillion plates you had to spin to keep your character going.
Don't get me wrong, I'd rather play something new. TBC isn't super interesting since I've already done it but the design philosophies of modern WoW are just so off putting.
I miss the days when you had your talent tree, you had your gear and you had your gold. That was all you had to worry about. If you wanted to do more you had reputations and professions to work on that ended once they were done. At the very most I think we should have a PvP currency like honor and a PvE currency like valor.