Preach is a veteran World of Warcraft player based out of the United Kingdom (specifically, out of England), and he's one opinionated son of a bitch. That's why I like the guy. Most of what he does are gameplay guides, but he does do retrospective videos like this from time to time and when I saw this come into my YouTube feed I had to mark out time to watch it.
He delivered. While I did raid and do PVP in Vanilla and The Burning Crusade, Wrath of the Lich King remains my favorite of all of the expansions to date. (Yes, over Legion, and if Legion keeps delivering it will surpass Wrath by its end.) However, what I think was the big draw is something Preach only hits on early.
Wrath of the Lich King is the end of Warcraft 3.
For a lot of people, they wanted closure over the stories left unfinished when Warcraft 3 ended. World of Warcraft picked up a lot of them, and The Burning Crusade direct addressed one of those loose threads with Illidan. However, Arthas and the Scourge remained undone and so when Wrath hit of course folks piled on to play. They wanted to tie up those threads, and they got what they wanted in no uncertain terms.
The massive dropoff with Cataclysm is because, for many, the story was over.
Yeah, plenty of other factors involved, but for many those other factors were just excuses to justify their decision to end their subscription and stop playing the game. Mists of Pandaria and Warlords of Draenor did little to bring them back, and (despite the facts) popular perception of Pandaria being a Kung-Fu Panda knockoff was a huge repellent while Draenor being a Burning Crusade rehash made little friends.
Legion, on the other hand, is bringing folks back. Yes, going back to the well (Gul'dan and Illidan) gets knocked on, but as other lore bits come out and gets talked up folks who dropped away after Arthas died are seeing that the something they wanted out of the game seems to have returned. In short, Legion took up all of the remaining loose ends (Turalyon, the Army of Light, Argus, etc.) while clearing out figures whose narrative relevance has ended and is going all-in on the storytelling.
That's something a MMORPG publisher has to take notice of: you need both top-tier gameplay and quality storytelling to succeed IF, and ONLY IF you want to make a Themepark style of MMORPG. That translates as Relevant Things to Do, Competent Mechanism to Do It With, and Satisfying Explanations to Do It At All. You need all three things, and you need them in proper balance, if you want to succeed in the MMORPG space as such a style of product.