It's been said more eloquently by a poster on MMO-champion, but the only reason I play current WoW is because I have an attachment to my characters and the world, which I love despite the debatable quality of the lore at some points in the story arc. Most of my characters (7 of them) are from early 2005, when I started playing at EU release, some are blood elves from TBC release (2) and then there are the death knight and the monk from their respective expansions. These are my 11 "mains", all level 100, split on the 2 factions (fuck segregated factions in modern wow by the way).
I usually enjoy the content provided by Blizzard and always did so, some times more than others. Since I was never a fan of RTS games, Warcraft 3 didn't influence me much back in the days (I played it very shortly and never the Frozen Throne xpack), because honestly I was devoted to Diablo 2. World of Warcraft made me fall in love with the genre, much more than EQ did, because it was just awesome. From February 2005 to April 2016 I have approximately 370 days /played, which pretty much means 80% of my gaming time. I've been subbed much more than I've been unsubbed (which happened from time to time for brief periods).
Even during modern WoW I had mostly to compliment them for the quality of their main title, for example during Mists of Pandaria, which I loved for several reasons.
Playing on vanilla servers is still a better experience. It's not a better gameplay, because the evolution in that regard is hardly disputable, especially for some class skills that made them more interesting and less absurd to play, but the whole experience is on a whole different level. The amount of dedication required for most things, even if done in little steps playing casually is alien to modern gameplay. The fact mobs could own you even during newbie levels (even pre level 10) was a testament to a world with dangers in it and as such much more interesting. Even 11 years later the journey (which I did several times already during vanilla) was very enjoyable, including the trips and the hikes around the world to collect all the quests to go inside a dungeon, such as the quest chain in Desolace for Alliance characters that ended with a quest for Scarlet Monastery (which rewarded an awesome 1h blue sword) or those chains that didn't have very good rewards, I remember the Faded Photograph found in Un'goro Crater, but it still connected different zones and gave the feeling of living in a coherent world.
I've never been a fan of flying mounts or instant travels or stuff like that, for the journey is everything and "end game" to me has always sounded like "ok the game is over, now starts the work". Raiding could be fun, but it rarely was unless you were so lucky to be a in a well coordinated and drama-free guild. I did MC and Onyxia and even BWL (and ZG, which I enjoyed a lot more, must have been the raid size), then I gave up after a little AQ20 and I never saw the original Naxxramas, nor AQ40.
I spend a little time on private servers, but I'd prefer if Blizzard would put up official vanilla servers, without fucking them up with some bullshit. Patch 1.12, it's never going to change, accept it or leave it: I'd pay, period.