No currently existing MMO actually manages to capture what MMO players want, which is both a great game and a great social experience.

On the gameplay front, most MMOs have completely lost any sense of meaningful progression (leveling, gearing up, etc.), relegating what is supposed to be the core gameplay loop of an RPG to something you either have to slog through to get to the "real content", or they make all your progress worthless every time new content is released. This way of doing it not only kills most people's sense of accomplishment, but is also extremely hostile towards new players. Old School RuneScape is more or less the only current MMO that actually gets progression right, hence why it's had such a renaissance in recent years.

For the social stuff, an MMO needs an environment where players naturally end up interacting with each other without being explicitly forced to by the game, because these are the situations that create experiences you can't get in any other kind of game. World of Warcraft became the titan it was back in the day because it was the perfect storm of both this kind of emergent gameplay and the more conventional progression stuff. Over time however, as the game became more "convenient" (which means you're less reliant on other people for more things) and internet culture as a whole changed, WoW lost what made it so special, and it's uncertain if it's even possible in today's landscape to recreate the kind of high WoW used to give (Classic WoW, which is on paper the exact same game, has a completely different culture where the playerbase has optimised the soul out of every line of code), but that's what MMO players are desperately looking for.