Eve Online accomplished something a little more combat-focused, but similarly diverse in playstyle, mostly by dint of having a single large persistent world-shard with minimal functional instancing.
What do you mean by this?
When UO came out, it had to deal with a large game world that nevertheless would be too small to reasonably hold a number of players that represented financial success. So the game world was duplicated on multiple servers running the same map. A player on Atlantic server could have no interaction with a player on Chesapeake server. If you wanted to play together you needed to make characters on the same server. In-game this was reflected in cosmological lore about the world being some kind of crystal that was shattered into shards. This jargon spread, to some extent, to be a generic terminology in MMORPGs.
I believe UO launched with only three shards, and added a couple dozen at the height of its popularity.
Eve is notoriously a single-shard game, with the player count being accommodated in other ways as the map grows larger and more organized play is added patch to patch, and the actual play changing notably from the era of 5,000 simultaneous online to the era of 50,000 simultaneous online.
Even without deep skill-stat specialization, when your alliance has 500 people it lends itself strongly to role specialization, because an alliance of 500 people is an organization of 500 people, who find different things fun in different contexts, and whose operations have every category of need you would find in a small nation state, from diplomacy to collective resource allocation to logistics to surveying to espionage to military service, recruiting, communications, practice and command.
I'd go a step further, not just equally compelling, but it'd be interesting to see some games, particularly RPGs, where combat is effectively optional. One of many ways to level up your character and complete the objectives of the game.
There aren't many out there where you could have a complete pacifist playthrough, for example, and if there is, you usually still have to resort to theft, or use of paralyze & calm spells.
In most RPGs your professions (farming, herbalism, mining, etc.) are just secondary skills to help you progress in combat, and all the good stuff comes from killing enemies.
