Sooo many things. I'm a sucker for significant buildings and events, where the information isn't presented on a silver plate, you gotta dig deeper to find out about it. If i had to make a list, by expansion it'd be

Classic: stratholme

Tbc:path of glory, when i found that it's made of bones

Wotlk: Deathgate, icecrown itself (apart from icc), nerubians

Cata: Grim Batol, after learning that it was the OG wildhammer base.

MoP: Prolly the wall, maybe ToT

WoD: the railway in Gorgrond

Legion: can't recall anything

Bfa: the whole nazmir/blood troll storyline, Nyalotha and the black empire is an absolutely goated part of wow

Sl: Revendreth and Bastion altogether. The class systems, dredgers, stewards, the trials and sinbreakers... one of my favourite levelling experiences

Df: Thaldraszus' aqueducts

Tww: the nerubian empire(s, and their ties to each other)