I don't think I remember many other things that were as bad as corruption. The funny thing is the closest in 2nd place was added in BFA as well... Azerite armor.
Maybe some close competitors...
Garrison? Hard to say that was bad, it was just that there was no other content to do in the game, as we saw in Legion when it was a supplement to the main content it was fine.
Um... flying? I think its fine now but an argument could be made it shrunk the world.
Phasing/CRZ might be a contender. Practically speaking, some servers were dead, but it also made the world feel less authentic.
Someone help me here.