I've outlined a couple of fundamental to a different kind of MMO - the character development strategy game. The first principle is a 24/7 experience system which levels the playing field between casuals and powergamers and offers the customer a sense that they are getting full value for their subscription buck; and the full linear and lateral availability of all character development avenues to all characters. Players will be able to spend their steadily-accruing experience in any line of character development. This development system also kills any chance for a balanced "end game", pvp, solo or grouping combat system.
Now the question is, why actually play the game? What is going to appeal to you to actually log in and spend time in the game? What mechanics can only be achived by being live?
Well, first you have to buy developments with your experience, and you have to be online to do it. Next, to see what those advancements do, you actually have to do stuff in the game. You have to be in the game to use your new spell on a mob, or to create a new piece of equipment, or to see what that new dance emote does.
Outside of just seeing the new effects, abilities, and gear, there's also exploring the game - just seeing what there is to see. There is interacting with others in a non-pressurized online game environment. There is of course "showing off" one's new abilities, characteristics and talents or acquisitions. Then there is fighting mobs, and performing tasks.
In this game, fighting a mob doesn't gain any experience, but it does gain loot. Mobs can drop resources that can be used, or cash. Mobs - any mob - have a chance to randomly drop rare rewards, even up to the best in the game (currently, anyway). There is no level-check system; an easily-killed bunny could theoretically drop a top level Axe of Doom (which happened to be half-buried where you killed the rabbit, to preserve RP). Different kinds of rare loot would have different drop percentages, so getting something like that would be extremely rare.
This kind of drop presents the character with a strategic dilemma; do they change their character development strategy to eventually meet the stat requirements of their newfound Axe of Doom? If you can't wield it without X strength and Y training in axes and Z understanding of Necromancy ... do you change your goals to accomodate this new treasure? Or do you just sell it? Or trade it? Or do you break it down to constituent parts and materials?
The other thing one can do online are tasks and quests, which can provide some extra cash and resources, but are mainly used to advance character development storylines. This game will have many in-depth storylines that advance along with your choices and are driven by interaction with quest NPCs. These character development storylines change how the NPCs and other aspects of the game interact with your character.
For instance, one of the advancment avenues is "profession", which is how a character earns in-game money and rewards while offline. If they pursue a professional path that takes them to being a diplomat, then at some point they are given a diplomatic residence in the province they are assigned. Further development via quests and advancment might gain one the opporutnity to acquire an assistant, and body-guards, and get discounts at the market, or have access to various parts of the city that others do not, or be able to commit certain crimes and not be thrown in prison (even if it lowers one's reputation in that province). One might become very influential and rise to being an advisor to the king and be able to arrange certain benefits for other players - such as a wedding celebration venue replete with full waitstaff, fireworks, stage, settings, furniture, etc. Or, they might be able to get housing or trade privileges for family or guild, or have access to historical or official documentation that become quests they can offer other players.
Similar storylines can be worked out for other avenues of progression which are "told" via interactions with NPCs via tasks and quests.
The lure of random, strategy-changing rare drops, gaining extra resources and in-game cash, and pursuing character development storylines and unlockable content via quest chains - as well as just exploring the game locations, mobs, graphics, etc. and interacting with others in a non-pressurized system - provides adequate motivation to log in and enjoy the game.
Next: It's the ECONOMY, Stupid.