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There are some wonderfully original, joyous virtual worlds out there. They're exquisitely balanced, rich in depth, abundant in breadth, alive with subtleties, and full of wise, interesting, fun people who engender an atmosphere of mystique and marvel without compare. Newbies would love these virtual worlds, but they're not going to play them.
Why not? Because they're all text. Newbies don't do text.
None of these principles are required for a MUD to be good, but the best MUDs tend to have them in high proportion.
1) Character approval. You submit your description, background and other information to a game master, and he approves or dispproves.
2) Permanent death. This is not notable except in its absence from MMOs. Traditional computer role-playing games and online ones had it (Kesmai, MUD1).
3) Enforced role-playing. No reason for a MUD not to have this.
4) Food and water required.
5) Challenge. Every MUD with permanent death has challenge that I have seen, other than Hardcore BATMud.
6) Roleplaying tools. This is what separates graphical games generally from text. Very seldom are graphics used for role-playing.
"I mean - you can't even *sit in a chair* for crying out loud. " - Kerry Jane on alt.games.everquest in 1999.
"Today’s graphical worlds are excellent at making a world look real, but as a consequence it’s harder for them to behave real. Characters jump into a river without making a splash, then swim across it in full armour without sinking, to emerge without being wet and with the glass of milk they’ve had in their backpack for several years still as fresh as the day they bought it" - Richard Bartle in 2011
7) Realism. Again not worth specifying. Graphical games stopped trying at all after the early days. Early MMOs had some examples of realism here and there.
8) Freedom to choose between a level or skill based system. The creator does, that is.
9) GMs enforce the rules and ensure people play by them. Careful study of player logs permits the GM to see just what a player has done.
10) In MMOs, players have limited power to affect anything.
11) RPP and roles. Even some of the light MUDs have serious player roles, whether tied to RPP or not. The early MMO Nexus (1996) had some of this and conceivably others that no longer exist. RPP can be tied to social class, wealth, magic, companions and other things.
12) MMORPGs began to limit which players you could attack, and to restrict looting of the corpse. This was standard by Everquests time. This was not traditional in MUDs, though it was not uncommon as they expanded in the '90s. It was unheard of on MUDs with enforced role-playing, as far as I know.
13) Alts are generally forbidden.