Edit: Yup +1 to Lyn and my expanded view on that -... (truncated)
Sort of this, but also sort of not. First of all, I wouldn't also associate 'time' with 'hard', though to be fair it is sort of the standard MMOs have been held to. I remember killing time by playing Minesweeper on expert mode. It was hard, but it didn't take long to play. It might be that MMORPGs kind of fell into an old RPG trap by saying that, if something takes 60 hours of playing to beat, it's a hard game. That's really not true, grinding isn't hard as much as a way to make sure you don't have everything in a game within a few dedicated hours by simply not having a life.
Having said that, I've worked steadily (minus a pair of layoffs last year) since I was 16 and worked full time through college. My game time became less copious, but I still found time to play. Maybe that's the answer to the problem, to just have better tools to log off and on and to make games simply difficult to play? Using the example of Monster Hunter from earlier (now that I've said it, it'll be hard for me to not talk about it), I can imagine a vast open world that it would be very simple to walk out into and pull a Les Stroud, surviving out in the wilderness by your wits, preparation, and whatever you brought with you. I can also imagine being able to get a mission from a town and that mission having a strict time limit. When you need to hide to run and take care of something, have a way to take a few minutes to hide yourself and if you need to log out, have a way to make camp.
Honestly, that's one of the things I think EVE does right in their mission structure. I like the idea that, yes, if I don't have time to do a mission, I still get a reward, but if I have the time and I book it to finish up, I get extra. That's a good way to both be flexible with people's schedules but give you something of a challenge to work on.
I think the thing missing from most MMOs is the element forcing you to work together. Usually, I think devs have fallen into the habit of making every encounter with another player a sort of PVP experience that you should be worried about. That doesn't necessarily encourage you to try and help random passerby out and make friends (one of the things I think EVE doesn't do very well on a fundamental level). In essence, I think one of the things that might make an MMO a more rewarding experience than a single player game is if players are encouraged to help each other and be polite while they do it. The focus on being able to solo play a game (thus meaning you can be a complete dick) and PVP (meaning you can dick someone over without necessarily being rude) in modern games does make it a lot harder to get the potential out of an MMO.
I made all my friends here, I never met any in the game world. Even most of my friends that I've met here I can't play in-game with because of its very competitive nature. I think that might be something Graelyn was touching on, as well, because devs have a raging hard on for faction-based in-game political systems. Which means you can't possibly win. You can only succeed on a very temporary level. You will never defeat the Grand Empire of Evilbastardstahn because there are players in that faction who you can't just remove from the game. It makes your game insanely inflexible. It makes PVP between those factions a complete joke because it's just an arena: you can't ever win or lose the battle.
I'm not saying PVP is always a bad thing or that those kinds of games don't have their place, but seriously, EVERY MMORPG GAME has this bullshit. You could solve that problem by having it go one way or the other, either having power be so loosely fragmented that every little individual town is like its own nation and to be so spread out that they don't have much to do with each other, or by having one giant government that handles everything in its area and sends you out to handle other issues.
Moreso, I think sometimes people don't know that it's just fun to break even sometimes. Maybe games, because it's how games have always been, are too process-oriented. Maybe we just need to have longer-term quest goals or to make exploration, discovery, and randomness more of a driver of game experience than the task-reward cycle. Maybe it would be nice to have a world so huge that, to get to where you need to go to kill something, you have to trek for two full days and survive all the perils of just getting there.
That's another thing! Why is travel so damn easy and turns games into a connect-the-dots story? Why can't 'getting there' be just as interesting and you can get sidetracked for days or weeks doing something because you happened to see something on the side of the road?
... I should probably stop. I could bitch about this kind of thing for hours.