Ever since I've played the first game, back on my ps1 in 1999, I fell in love with co-op and multiplayer. Few years later internet came and the whole new world opened up for me. Since then I've played countless games like Cabal, Drakensang, Grand fantasia, Fiesta, WoW, Guild Wars, LOTRO,etc... I've played a huge number of mmo and I'm still playing wow (since 2005) and this is why today's MMORPG suck.
Whole MMO scene really sucks today. It's interesting how most of the games that are coming out today are mostly some asian ports (God knows why they think those games would work with western audience) or really not approachable games (needing 2-3 IBM supercomputers to run with the recommended settings) and I believe that's the most important part. They all have shiny graphics, cool art, Unreal Engine 8 from the future, etc... but, let's be honest, those games are plain and boring. Why there will never be a game like WoW? For a few reasons:
First is accessibility, a game that even toasters can run it well (WoW) and more people can enjoy it. No need to require 16GB of RAM, latest GPU, 16 core CPU,etc.. Not everyone has $10.000 PC.
Second is background of the game. WoW had a huge advantage because of the warcraft series and most of the games today push their own story based on nothing and majority of the people won't even bother with it, even an mmo where you'd be playing as a miniature version going around a casual house and backyard fighting ants and cats as raid bosses would be a lot better to build upon. That's why I believe that if the RIOT was to make an mmorpg based on League of Legends universe (same graphics and all), they'd get quite close to wow now.
Third is the individualism. Why on earth do you think that classes that can be all classes and do everything as all other but have different spells is a good idea? It's beyond stupid. People want to be able to stand out from others, so tomorrow you can say "Yeah, know that guy he's an excellent priest/healer" or "invite him, he's one of the best tanks I've raided with". This goes with today's mentality, where everyone get's a participation trophies and no one's being left out (also Holy Trinity is the law in rpg). And why would anyone want to play games where you lose your hero after some time or something like that. I don't want realism in games where you can build houses, die of old age, be classless on the start,etc.. I do all of that stuff IRL. When I play an mmorpg, I want that character to represent me and he is representing me in that universe. (skyforge)
Fourth is making spells shiny and flashy. When I press one button that's for a spell called "fireball" or "stab", I don't want to watch my character jumping around and spinning for 15 minutes because I've pressed one button to cast one spell or that one of the most ridiculous things even implemented, a white something that's following the path of your weapon.. oh my god, don't ever to that (dauntless).
Fifth is gameplay itself. I don't know if developers of some games ever played and GOOD mmorpg or any for that matter... being able to tank and kill 3-4 mobs as a rogue or mage on the very start of the game is ridiculous. When you start a new character, you're nothing and nobody until you build a name for yourself. I won't go any more further into this topic.
Sixth is the difficulty. Auto-walk to the quest giver and quest objective, pointers on everywhere you need to go, being able to solo everything, quest text is not needed unless you're interested in the story for some reason,etc.. stop treating player like they're spoiled brats.
Seventh is the optimization of the game. There's been a number of times when I, obviously, couldn't run the game on high or even medium setting so I had to lower it down to low... Every game is horrendous, if it's that badly optimized then it shouldn't exist at all. I have a PC that's on lower end and I can play wow even on medium settings while pushing 60fps easily and the game still looks amazing... I can't do that on any other game (wildstar, neverwiner, warframe,etc...).\
Eight is the leveling/end game. Today it's all about end game experience, my most beloved memories are from leveling my tauren warrion in World of Warcraft for a year
There's probably more but I really need to study right now, and before someone replies with:" That's just you man, someone thinks >insert low populated mmorpg here< is a great game", well something thinks that nazis were right and if those are such a great games, where's their player base? Any "good" mmorpg today can fit in multiple of what I've said.
MMORPG developers will never understand why was WoW the biggest mmo and why it still is and will be.