For me:
1. Lower barrier to entry. Most games understand this now. Most understand that you've got to hook your player within the first couple hours if you want to have a chance at keeping them. Again, WoW really mastered this. I remember going from the last MMO I'd played to WoW (which I think was Final Fantasy Online), I was stunned by the amount of shit that felt like it was just handed to me. Levels, gear, quests, abilities, money, all within minutes. There was so much going on that between that and WoW's 10 ton visuals (I started as a Night Elf), reading the quests to see why I was actually doing all this seemed pointless.
2. A preference for actual live zones versus instances. This is a HUGE reason why community took a hit in more recent MMOs. The original concept of MMOs was thousands of players sharing the same space, literally. Then lessons were learned about the performance hit that creates, how the infrastructure of truly popular games can't handle millions of players in the same space simultaneously and how people generally turn into dicks given enough time and opportunity. When everyone was fresh to MMOs, they had to feel their way around. They had to talk to people. They had to ask for help. A lot of game knowledge was stored in player's heads, not in robust wikis. But after the downsides of large shared spaces were recognizing, instancing happened. The social aspect became more and more abstracted away for the sake of performance and clean, repeatable experiences. (Doing a dungeon run in EQ was an utterly different experience than doing one in WoW.) Queues were created to handle player loads and make sure everyone got a group to so no one was left LFGing their life away. But you don't need to know anyone beyond whether they suck at their class when it comes to queues. They might as well be an NPC for all you actually need to know them. I miss the old days of 20 or 30 players in the same dungeon trying to make their thing happen. It's what made game worlds feel truly dynamic to me. (Even though it made it really difficult to progress at all and lead to preposterous behaviors like camping the same group of monster spawns all night instead of actually progressing through the dungeon.) Still, every day felt like its own adventure because every day brought different people and different things happening, and that was almost enough to make up for it being way harder to complete anything or see it through. You were Chasing The Dragon.
3. Getting away from the WoW model, in general. In some ways this means getting away from a lot of the conveniences it introduced. A new way of doing gear, no more common/uncommon/rare/unique/epic/super epic with a nice color coded system. Combat mechanics that are actually original. Not doing the "Rolodex of Quests" per zone. Not having cleanly laid out, easily identifiable quest givers with brainless quests so you can meet the level threshold for your zone and then progress on. I really want loot that I value for it individual qualities and not just because it has 35 more attack, 2.5% more crit, and 13% more crit damage. Every fucking game I play today uses these rules it seems like and it creates a sense of boredom for what is supposed to be the biggest carrot in these games. I want gear that I'll actually keep with me for a while, not gear that is literally so replaceable I can't even remember the name of it or what it does without going into my inventory. Seriously, we're not kids anymore. We don't need to be stimulated every 2 minutes with a new loot drop to keep us interested if the game itself is actually enough to keep our attention.
4. Respect for my time. That kind of goes along with #3. So much of WoW set up busy work that you had to get through to do anything interesting. Oh please farm 40,000 rep to buy this key so you can go to this dungeon. Oh please do three different difficulties of this dungeon before it's complete. It is a game company's necessity to keep you paying your monthly fee, or buying microtransactions and game cash, that has taken the original model of grinding and expanded into this elaborate, never ending cycle of trivial tasks meant to fill up our day. Why create new and truly interesting content when it's far more cost effective to just take the same thing you had before, tweak a couple settings and just invent a new tier of busy work to occupy a person's day, where there is no real mystery because there's nothing really new other than higher level monsters and better loot. You're not so much Chasing The Dragon as Holding On To The Dragon's Tail While It Slowly Drags You Through The Game.
5. Real danger in everyday gameplay. I'm advocating for a return to real punishments in MMOs. That may be in conflict with #4. But when you trivially die and respawn all you're left with is the time you've lost and the knowledge you've gained, and I've done enough raiding to know that knowledge alone can't sustain interest when you still can't do anything with it. Real penalties that makes adventuring dangerous is what gives a world character and spice. It's what invests those mobs with personalities due to the makeup, instead of just making them a collection of stats, AI pathing behaviors and loot tables. We have become so risk averse in MMOs due to how we value our time and how we just desperately want to progress that we've come to accept a tepid gruel of same-y trivial content spoon fed to us for the vast majority of the game. Only when you start raiding does shit really matter, because raids are truly designed to be challenging. Lots of people are like "oh if you want danger play PvP. PvE is for my safe, bored solo farming." Plenty of people enjoy SP games with real element of danger to it. MMOs became to afraid to alienate their customer base to try that anymore, and it's why I can't take a lot of them seriously.
6. This may sum up or contain all the other points. Make it about the journey, not the goal. Thanks to WoW's loot system and idea about linear progression, we no longer really give a shit about what we're doing, we only care about what it gets us. It's part of the reason the social aspect died, it was relegated to "Can you help me achieve this thing I want? No? Then I don't need to talk to you." Who has time to talk to people or get to know them when the next quest step can be sprinted to in 30 seconds, or the next dungeon mob should be pulled as soon as the first is dead. The quests that get you to the "big moment" should provoke thought, challenging your skills and encourage exploration. If all we care about is loot and whether or not we're keyed to go on to the next thing, then half of what makes up an MMO game just feels like busy work we don't pay attention to. It cheapens the lore, it cheapens the experience and it makes your players jaded about future content because they expect it to be more of the same, just with a different coat of paint.
Here is I think my real problem, something I associate with writing ultimately: how gameplay is designed follows the formula first and foremost and the ideas that underpin it come second.
For example: mmo devs are about to create a new zone. Immediately they apply the formula. It has to be this big, have this many dungeons, this many cities/rest points and hook up to these different pre-existing gameplay systems like factions. Ok, we've plotted these hard numbers. Now what is the zone actually about? Well, let's make it a fire themed one. Ok, so what lives there? Well it's fire so.....dragons obviously. Why are there actually dragons in this zone? (Lore writers spend a while concocting a backstory for dragons in this world.) Ok so what are the dungeons about? Well obviously there's dragons in the zone so....dragon dungeons! (Lore writers spend time concocting an explanation for why there is a giant castle on top of a volcano where the Dragon King lives and why you should kill him.) Oh we need a lot of subfactions to populate the zone and hand out quests. (Lore writers come up with a dozen minor groups that have varying reasons for existing, but mostly so you have someone to give you quests and sell you things.)
And on and on and on.
Players recognize this shit almost the moment they see it because the game has been training them to expect it. For me, as a player, that pretty much instantaneously steals the thunder of whatever exists in this zone, because I know its existence probably isn't a flash of creativity or awesomeness from a game designer, but a work-a-day requirement to create a thing and fill it with stuff, and the easiest way to do that is to NOT have to re-invent the wheel every time, which a good formula facilitates. But by making their work easier and content creation more seamless, it loses that creative vital spark which makes something truly special, weird, outstanding or interesting. It's a pipeline of content creation so everything needs to have the same general shape so it doesn't get stuck in the pipeline.
There are examples of this in WoW: Kharazan was great. It was new and different than other raid dungeons before it. A great combination of layout, design, new mechanics, adjusted raid mechanics, aesthetic and details. It's awesome in a way Dead Mines is awesome for the love you can feel went into it. Or the Scarlet Monastery. Or Stratholme. But compare and contrast...fuck, the dozens and dozens of dungeons you vaguely remember, whose names you don't give a shit about because their awesomeness and creativity didn't sear itself into your gamer's brain. A lot of the Burning Crusade dungeons are like this to me.
Where are the zones you can't explain? That don't fit into the formula? That exist because they're cool and not just because it's an option for 20-30 leveling? I remember in EQ one zone was just a mansion. A big haunted spooky mansion, where each part of the mansion was laid out as its own area with its own monsters and stuff (Estate of Unrest.) It was offbeat in a game that was largely comprised of open areas with a smattering of dungeons and NPCs around. Or a zone that was nothing but a mountain pass loaded with enemies, with a town at one end and the zone exit at the other, and the town was regularly "under attack" because of PCs training monsters into it as they passed through (Highhold Pass.) These places had character. Their very design was their character and everything else built on that. In WoW HHP would be jam packed with vendors, faction reps, quest vendors, 3 dungeons and miles of useless canyon for the express purpose of not seeming to small compared to other zones, and waves of monsters would regularly spawn just to get slaughtered on the front rank of town defenders so players would see it as "epic" that there is this constant (and completely trivial to gameplay) conflict going on. I'm not going to say that Everquest didn't have plenty of clunker zones that were uninspired, too big with nothing in them, etc....but its high notes were always so much sweeter than WoW's. I'd say the only thing WoW does that is undeniably better is interesting raids, because it seems like that's where people get actual time to think and create interesting stuff. EQ had some cool raids for sure, mostly due to the level design, but the fights aren't as intricate in their details and stages as WoW raids eventually became.
In the end I just want the craftsmenship of ideas and content to be a thing again, not just how many hours an MMO can occupy you or how many "quests" it offers. It having a Diablo-like loot system should not be a selling point. Variety, surprises, a formula for design that knows how to keep itself hidden, and a game that actually asks me to think about how to play it are all what are stopping me from getting back into or trying a new MMO. In a way I think new MMOs are basically just fan service. WoW created a sustainable, constantly sellable model for MMO design that everyone picks up and adapts to their own particular genre or IP. The D&D MMO, the LotR MMO, the Marvel MMO, the FPS MMO, the Conan MMO, on and on. In a way WoW is responsible for making all these games even possible because everyone who made them already had an idea what to do going in and that made them a lot easier to be greenlighted by publishers because it was a model that had already succeeded big time for Blizzard. And yet, many of these games are no longer around because they couldn't go the distance. And those that tried to be different I suppose just weren't awesome enough to survive where Blizzard is still King of MMOs.