Practically all MMOs are built from a similar tool kit and thats why they are boringly similar.
The plot is always moot, because you are doing these world saving things along side a bajillion other people, which kills the individual feel. You never have a chance to change anything or do anything unique.
To be honest, the plot thing is the killer to me. Not all games can be wonderfully innovative with mechanics, but a good plot makes up for it. But MMOs don't have plots, they have fetch quests and kill quests and not much else, and no matter how you mix that up you never feel like you achieve anything in a plot sense.
So basically you are left with the mechanics of it, and they almost all follow the 'trees of abilities and unlocks' approach, which makes things very dull to me. You just press the keys and stuff happens. You find equipment, and you use it *shrug*. And they are all exactly like that.
The only genuinely different MMO is Eve Online (which I've played on and off for years) and while it is definitely different, it'll always be a very niche game that is grindingly hard to get into, and takes a well groomed sense of masochism to stay with.
What would get me interested in a new MMO would be something non-fantasy (which has been done to death and I never liked that much anyway) that offers me the chance to play solo if I want and the plot makes me wanna stay with it. I don't know what that means in practice, but I want a quality single player experience. Not just 'stuff to do while I level up' but something that actually makes me feel a part of something.
I'm thinking something like mass effect, and the MMO aspect is a squad based thing. A big universe and I can be a solo agent doing something, or part of a squad or part of an army. Dynamic scaling to the number of players you have wanting to play at any one time. I'd organize it so that you have big areas, that mostly just have NPC enemies (for both sides) and then people of either side can leave or not and it scales the number and relative strength of the NPCs balances out. Basically so that you have a reasonably fair fight all the time, and you can PvP or not, but even when you do, you always have back up. That'd mean that a newbie team could still fight against super veterans, and the NPCs help make up the difference. The vets get low level grunt back up, the newbies get smart, team focused dangerous NPCs. It'd still be an up hill fight, but I think you get what I'm saying. Maybe add a tactical role too, so someone can co-ordinate the NPCs.
That'd work for me. Maybe some kind of planetary war like in Planetside. A solo part that is say 15-20 hours long, at the end of which point you pick sides in the war, and then start fighting. The war can't truly be won, but there's a genuine back and forth, and even if one side is dominating, the game makes their stuff harder and harder until the others push them back.
Thats basically what I'd like to see. More sci-fi, more good solo parts, less 'level and class' kinda crap (seriously I hate it) in favor of a system more focused on equipment and your personal skill with it than stats vs stats stuff. Make it first person and shooty too. I hate how practically all the MMO have zero personal skill. Balance that out by never putting too many players vs each other so its not too death matchy in pvp.