Talent trees or class design for me.
Proper dailies instead of world quests for me. I miss you, fishing dailies. I never thought I’d say that.
Zone design.
I can actually see the skybox in BC and there isn’t a mob every 5 feet.

Pretty much nothing
Dailies wrapped up in interesting rep quest chains. Callings and World Quests just seem so pointless and shallow.
Oh man, only one thing?! There are too many to choose just one. What I would leave in retail is a much shorter list!
Karazhan 
flying with just a gold price along with zones designed for flight
The lack of mythic plus for me.
If we are talking about TBC original release, then all of the friends who used to play back then. 
Kinda how I feel too… TBC is no paradise, it has plenty of downsides too, but it’d be easier to patch up into something nice than SL would be.
the community.
I agreee. Talent free was mucho fun
Quest design and optional dailies. I also liked attunements.
Realm community and not this CRZ garbage we have now.
I’d also say death to sharding as well.
Not a thing. TBC was not very good.
Straightforward game design instead of 25 overlapping systems that destroy the game.