I'll just echo what has been repeated here: None.
The last MMO I played for any significant period was Firefall, for reasons discussed to death. Dabbled in a bit of BnS, loved the art and combat, but the questing system was absolute dross and I dropped it. Tried a bit of Tera; again, enjoyed the combat, but the questing was your standard fare, dropped.
Honestly, I have lost faith that there will be an MMO that truly innovates and brings an alternate world to life. Developers/Publishers/Suits, play it far too safe. One should not blame them too much, MMOs are a massive investment, and the risk of failure is too great. And yet, the MMO crowd has been fed the same standard fare for so long, anything, any form of innovation, would be welcomed with open arms.
Since there are no MMOs that I look forward to, I'd rather address the second part of the discussion here: What do I think makes an MMO good, which I will get to by listing things that I hate about current MMOs.
1. A Discovered World: The first issue I have with MMOs is that everything is defined and set up for you, the player, to come along and challenge it. All I need to do is open up a map and I have everything laid out for me. From the shop next door, to that top-secret-bigevil-that-has-not-been-seen-for-millenia-and-just-awakened-in-time-for-you-to-give-it-a-proper-beatdown, I can see everything the world has to offer. Immediately, the game no longer exists as a world to explore and enjoy, it becomes a theme park, where I start to prioritize which quests I want to tackle, so that I can maximize my xp gain. Move from point A to B to C, preferably fast travel, so I can avoid all the 'unnecessary' content in between, because it's not worth my time.
Give me a world to discover myself. Not every bit of content needs to be highlighted with the biggest exclamation mark you can code in. Give me mini bosses to stumble into, powerful items that are not tied to a quest chain, hell, even some crumbling castles with unrecorded lore tied to it. An MMO world should be a getaway from the monotony of real life. The more you repeat the same thing, the more it becomes a chore. Allow the player to find things for himself, with each new bit adding to your understanding of the world.
This was one reason why the SiN Imprints in Firefall were such a joy. It gave you an insight into the world of Firefall, but not all of it, just enough to get those gears churning. Leaves you wondering about the history of the world. Makes you want to find the next piece of the puzzle. Such elements gets you invested in the world. That feeling of a kid on christmas eve, unwrapping presents, and with each piece of wrapping that comes off, your excitement grows. I'll take that every single day of the week, thank you. I'll point to Horizon: Zero Dawn (not an MMO, I know) as a masterclass in storytelling. You start in a world you do not understand, and each step, each item you find, adds and unwraps a new layer. Straying from the beaten path rewarded you with more lore and more content to sink your teeth into.
Is this easy to do in an MMO? Hell no. You have a world with thousands of players running about, and no dev in the world can create endless content. It is simply not sustainable. At some point players will discover everything there is. All the tidbits, all the secrets, will be laid out for easy access on the forums. And you eventually fall into a cycle of running from point A to point B with nary a thought in the world. But, what if you had.......
2. Consequences: Oh bother, we failed to beat that boss in time, got wiped.....Eh, no worries, it's gonna be back next week, same place, same time, until then please enjoy the regular fare. No, just no. I want a living, breathing world. MMOs these days are far too static. Players know exactly what boss spawns when and where, what NPC to find and where. The core issue is that you, the player, cannot screw up. Everything is static and permanent. And that is mind numbingly boring.
Instead, how about, oh look, you just delved into a cave, fiddled around with some arcane runes, and voila, you have just released a vicious demon that was sealed away. Fail to defeat it, and now you have a demon running rampant and attacking towns, NPCs, players that wander too close to the cave. Defeat it, and you are good. If not, the demon continues to grow in strength until you have an archdemon that is now a global threat.
Does everything need to be a world changing event? Absolutely not. Failed to protect a town from a raid? You just lost access to a valuable NPC that sells Thawing potions. This then creates a shortage of thawing potions, making the nearby boss fight all that more difficult. Is it a world-ending event? No. But what it does, is mix things up. It affects the economy: you now need to trade thawing potions between each other; it affects gameplay, enemies in the area are tougher now. One small event with several knock-on effects.
What this also does, is make the journey important. The target is no longer simply the quest chain. You have distractions now. A new event popping up, a new quest available cuz somebody failed at something. And immediately you find that the 'available content' has depth. There is a limit to the breadth of content that can be created. But a world with consequences can have amazing depth and interactivity. But all that means nothing, when you have....
3. The Massively 20-man-instance experience: I won't lie, I am not a fan of instancing. Instanced bosses that pop up on a regular schedule, with a fixed loot table, and the elitist hardcore culture that it cultivates, is not my cup of tea. Players eventually find the perfect team to run with, and interactions become limited to that group. Of course, I can see hundreds of players in the hub area, but how often do they interact with each other?
Fixed spawns/ timed events introduce routine into the game. Eventually the game devolves into a waiting room for these 'events'. And when you do play, it's with the same 10 players you have played with. A world with instanced/gated content is a static world. Gameplay becomes a series of running X dungeon Y times, until you are geared and levelled enough to do Z dungeon, rinse repeat ad nauseam. And you couldn't blame players. They are always looking for the optimal path to endgame. The problem here is that the developer enables such gameplay with their design decisions.
Content needs to be developed with the intent of bringing players together, not segregating them into multiple smaller playboxes. Encounters or event needs to prioritize the social experience. MMOs need to move away from being Massive Multi-sandbox Experiences into true Massively Multiplayer Experiences. And a true Massively Multiplayer Experience cannot be achieved when you have....
4. An Inconsequential Journey: It's always an exhilarating feeling, starting off as a noobie in the main city of this brand new MMO. You are armed with barely a stick, clothed in rags, and vulnerable as can be. You go out to kill a few rats, level up a few, so you can wear better gear, equip a better sword. And as you step out into this brand new world, grow in power and improve yourself, you start to leave the world behind. After the first few levels, everything, the world, the NPCs, the story becomes inconsequential. There's always a certain threshold past which you are flying past levels, dungeons and content, never stopping to look around. At that point the player becomes laser focused on the endgame, the ultimate weapons, the ultimate gear, and everything you do is driven by the sole aim of getting to that 'endgame'.
Players chew through content when they have that fixed goal in their sight. A developer can create the most fantastical landscapes, craft the most wondrous lore, entertaining NPCs, exceptional encounters, but all that is left by the wayside, once the player 'outlevels' them. All of them are just another stepping stone towards endgame. Current endgame-based MMO design has created a subset of 'wanderers' that move from game to game, finishing off available content and moving on. This does not help with player retention, or player investment. They are simply around long enough to tick something else off. If all your 'good' content is in endgame, then you run the risk of players rushing through the game/world, finishing off endgame content, and being left with nothing to do.
Instead, give players a reason to return to old content. Places, cities that you pass through shouldn't just be markers that you cross off on your journey, they should be constants that keep you coming back. A world with consequence, going back to my previous point, would alleviate a lot of this issue. When something that happens in the starting city has a direct bearing on what happens on the frontlines, the player has to 'return' and involve with the NPCs and places he journeyed through. It creates a world that is engaging, close knit and interactive. And as a result, all that time invested into crafting the world, the NPCs and the lore become worthwhile. It also encourages developers to pay attention to the content that they develop. Effort needs to be taken to produce good content across the board, not just the endgame.
Now, I'm quite aware that all that is not easy to implement. It requires a re-invention of everything MMO. Everquest Next had some good ideas. It actually had my interest for a while, until they decided to cancel it. Was it difficulty in implementation, risk involved in such an undertaking, or fear of failure, we may never know. But if you want me to get invested in a world, you need to create a world that is brimming with life. Sadly, none of the MMOs out today have shown even the slightest inkling of even trying to innovate. A turd log coated in gold, is still a turd log. Chew it whichever way you want, it still tastes like shit. And honestly, that's all that is out there these days.
Well, that was a rant and some. And I feel there is still more I can add to the list, but I'll stop here.
P.S. Reading through all that, all I can think of is: Damn, I miss Firefall and what it was shaping up to be.