I wish someone who has a good memory of these things and was there, would document that the key game design features of a DIKU are, if everyone is going to refer to MMOs as DIKU derivatives.
— a comment from Daniel Speed on Broken Toys » Wikicrap.
Glossing much here… Edit: this article is getting updated on the fly as people add comments and reminiscences.
DikuMUD was derived from AberMUD, which was similar mechanics, but had more of a scavenger hunt mentality in some ways.
At its core, it is a class-based RPG with the principal classes being fighter, healer, wizard, thief. (Later codebases added more). It was heavily based on the combat portion of Dungeons and Dragons. Advancement handled by earning experience points through combat, reaching a set amount of points, returning to town and “levelling up,” which unlocked new abilities. Classes were immutable (though eventually systems such as remorting, etc were added). Rewards for killing things also included equipment, which affected your stats and damage capability. If you reached the maximum level, common cultural practice was that you were invited to become a game admin (this practice dates back to much earlier, and existed in some form in MUD1).