The reason a lot of us loved BC so much is that it’s when Blizzard finally got it right.
Vanilla was terrible, let’s be honest. Talk about a lack of content. BC was when the game was still fresh and new, but there were things to do and everything worked for the first time.
(Vanilla is bottom on my list just because it’s the only time in the game’s history that I unsubscribed and quit playing. Didn’t come back until a few months after BC released, when my RL friends told me the game was actually good now.)