Given how diverse and personal the concept of "this is fun" is, this is a very difficult question to answer.
Since people have already voiced opinions about "short-lived" modes (ex: L&S) vs. "long-term" (ex: Steam Fortress), vs. "Career" (ex: WoW), I'll take a stab at my usual verbal diarrhea on the following topics:
- Directed vs. Aimless
- Individual vs. Communal Achievement
- Proposed game mechanic: Maslow's Marshmallow Pyramid Scheme
- Proposed game mode: Grand Civilizations, and their overly demanding citizens
Directed vs. Aimless
I prefer directed games, as in there is some or the other ultimate goal, even if it is a fuzzy one, or a massive undertaking, or utterly nonlinear (i.e. a list of many goals that are to be achieved in whatever desired order).
In fact, a nonlinear list of multiple goals I feel is the best, as it allows the player creative freedom, while providing something to aim towards.
(Minecraft Complete the Wool Monument maps such as the Super Hostile series is a good example of this).
Individual vs. Communal Achievement
I like the idea of a game where the player isn't just some lone hero somehow toppling every insurmountable odd with their shiny sword of smiting +74123 and polished mustache. But instead forms part of a community or team, working together to achieve greatness, while still having flexibility for the individual to shine.
A powerful/advanced individual should be able to take on many weaker foes, but not endless weaker foes.
Similarly, the effort of the "I am powerplaying 20 hours a day" of an individual should benefit all it their community, though still benefit that individual more. In a way, think of a "not too plagued by human flaws" economy. If an individual manufactures a lot of goods, it benefits the community as a whole (even a newcomer can have access to the new goods), and the individual that managed to achieve it reaps the additional benefit of making a lot of money.
Maslow's Marshmallow Pyramid Scheme
A great many games suffer under the problem that the beginning is hard, and as you progress it becomes easier. This boils down to the player usually becoming better (through whatever means) at the game, while the challenge stays much the same.
In minecraft, this is especially obvious in things such as food and resource gathering. Once you have really good tools and infrastructure, resources become near meaningless.
To this effect, I suggest that the challenge should ramp up.
I.e. monster attacks should grow more severe, etc, etc. and all those other means as well.
But specifically here I think more of the player should have something similar to Maslow's pyramid.
Initially, a player should desire drinking water and food, and simply what is required to survive at that.
But as the player consistently satisfies those needs, the next tier of the pyramid should start gnawing at them.
The need for clean drinking water, then more food, then shelter, then more variety of food, then luxuries, etc. should kick in, each a step in the pyramid, and each new need rather stubbornly staying in place even if the player then starts failing to meet those needs (increase in lifestyle is easier than decrease).
That way, the player is forced not to just spam wheat and bread to render the hopping hunger bar meaningless, but rather has to actively explore, and upgrade, and expand to meet their own greedy desires.
A player might need to decide to exercise restraint and not satisfy a new need, until they are truly ready to, and instead suffer the unhappiness (and whatever game mechanics that might entail) instead.
Grand Civilizations, and their overly demanding citizens
Now, on a similar thought line, the same idea can be applied to a civilization.
The ultimate goal being to take control of a civilization (i.e. gather minions of a specific race, or various races, or whatnot) and start a community.
That community will also have a pyramid of needs, starting off with basic desires in small quantities and increasing to complex and advanced needs in large quantities.
The player has to then meet those needs, via automation, commanding the minions, whatever it may be.
And again, the civilization would be more eager to increase their life styles than decrease it.
And one could throw in increasing disasters and monster attacks and whatnot as the civilization grows (via their own wanton raping of nature or angering other civilizations or whatnot).
This would also allow a 'teching-up/levelling' mechanic that benefits all.
Say, instead of a player building their own little furnace for smelting steel, they have to build a more expensive steel mill that can be used by anyone in that civ to make steel, and is used by the civ NPCs to make steel all the time. Those that contributed to the steel mill construction gain some benefit, but everyone can then use the mill to obtain steel, and not have to make their own. (Though perhaps need to expand it if the demand gross too large).
Or say that some players create a magic school, allowing them to learn better spells and abilities. As the founders, they get more skill/attribute points, but all can still go to the school and learn magic.
Etc. Etc.
One could even add cultural interactions, where you need to maintain a position of power within the society, otherwise you can no longer directly influence the society's choices.
The final goal could be a massive one, such as growing the civ to massive proportions of great achievements, in all forms of tech and magic. Etc. Etc.
Last words:
This, again, is more my style of play. The civilization type goal. It allows for communal goal driving, with individual achievement. It allows scaling of difficulty with the player's growth. All in all, I think it could work rather well.
You could even allow a 'semi-reset' approach, where once you 'beat the game' you set off with a small handful of your civ, some resources, and go to create a colony in a new, perhaps more harsh, location, while now having to meet the 'tribute demands' of the capital while meeting your own needs.
Stuff like that.