Ed Greenwood's Forgotten Realms has quite a bit of depth but the way it is presented still lacks appeal to me. MUDs are only one of a variety of games. The same goes for Sim-game like SimCity, Civilisation, Caesar and Lord of the Realm. They each offer some interest, but these are commercial products and are single-player. This is where I put down everything I want from a game.
MUDs have multiplayer in spades.
Generic fantasy worlds bore me to tears. The environment is dynamic, cause and effect operates and the world evolves. Sim games have a measure of dynamism. Ultima Online and other commercial online RPGs have AI models which offer plenty of scope. The environment is dynamic, cause and effect operates and the world evolves.