The other part of it is a genuine desire for a more difficult, time-consuming, socially-driven game.
I was a pretty dedicated player, off and on, from open beta in late-summer 2004 through the Cataclysm expansion in 2012, with a little dabbling later for Mists of Pandaria in 2014.
The game changed a LOT. I was never much of an endgame raid player; I really, really enjoyed the progression through the world with friends, so I did that several times.
As WOW shifted to be more endgame raid focused, world progression got a lot faster, a lot lonelier, and a lot less interesting. OTOH, those first years of playing WoW remain, to me, probably the best video gaming experiences I've ever had (and I'm pushing 50, so I've seen the whole ride).
I'll probably re-up to play some Classic. I doubt I'll stick with it -- I have other things I enjoy doing now that would preclude me from playing more than an hour or two a week -- but it'll be fun to visit that world again.
(Oh, and I should note for non-WoW people that the actual in-game world literally changed -- geography shifted, towns were destroyed, etc -- with the release of Cataclysm in 2010. Among the shattered places left behind were the early-level zones for my first characters. I'd love to see those as they were again.)
I never played original WoW, I joined in TBC but I really want to try Classic and I expect that I will love it. Nostalgia has nothing to do with that, obviously. Also, if we're talking about nostalgia, it's WotLK for me. And I'm not going to play it again :) Been there, done that. And a lot of players tried to remake classic WoW on pirate servers and they did amazing job, actually. Hundreds of thousands of players played on private servers for years. So demand is there.
Yes, pretty much. The reason it has been getting so much fanfare is that people have been trying to play vanilla WoW for over a decade on private servers, but kept getting shut down by Blizzard's legal department. Blizzard refused to host a classic server for the longest time leading to the infamous condescending quote "You don't want that ... You think you do, but you don't"