I'm surprised Bartle neglected to mention the most incredible thing about MUDs, which to this day has yet to be surpassed by even the most advanced MMOs. Once you reached the maximum level in MUD, you became a "Wizard". This gave the user access to the filesystem, and code-slinging capabilities. Since MUD runs an interpreted language
, edits to the codebase could be seen immediately, in real-time, to the users. Wizards could essentially change the world as it was being played, creating endless opportunity for creativity and spawning some of the coolest MUD "worlds" imagineable. Of course, this presented a whole set of issues with ethics, balance, and cheating, but those got resolved by the best MUDs over time.
Imagine if the next generation of MMO, or Diablo 4 or whatever, allowed for the players to graduate to "Wizard" status and create new worlds for people to play. Maybe then, these games could still be played over 20 years later like MUDs.