What keeps players coming back to an MMOG.
In order of importance: Community and Content (and potentially any Creations you make within the game).
Different players are interested in different things:
Hearts, Clubs, Diamonds, Spades: Players who Suit MUDs
(Note: 'MUD' (Multi-User Dungeons) is the term used for text-based online virtual worlds that MMOs developed from)
There's also more advanced Bartle MMO-player social categorization scheme that subdivides a cube into 8 chunks, rather than a square into 4 chunks, because it adds a third axis.
To every suit of players, people you know in a game become one of the motivations of staying with a game even past it's normal 'enjoyability' factor, but it might not be the primary factor.
To socializers, other humans are content. Whether guild socializing, or just chatting with strangers in the town square. The more people are willing to talk with them (and especially, listen to them), the more valuable the game is.
To achievers, challenges are content. Both number-based achievements ("Reached 1542 strength!"), and skill-based achievements ("Finally beat that boss, without wearing any armor, while it was raining outside, and got my Grinder Of The Year achievement unlocked")
To killers, prey is content. The more the prey gets irritated or upset, the better.
To explorers, the world is content. Not just geographically ("Reached the mountain top, and found some cherry blossom trees"), but also mechanically ("I found if you mix cyanide with orange juice, it nullifies most of the toxins.").
Players can be a mix; they don't have to be one extreme or another, and they may have other interests as well.
