Craft-based MUDs
Greg Costikyan describes A Tale in the Desert as a different MMPG:

Essentially, the core mechanic of A Tale in the Desert is a materials-processing tree tied to a tech tree and a list of skills. You gather basic materials to build gizmos that let you transform the basic materials into more complicated materials so you can learn more skills and build more gizmos and even more complicated materials...

This sounds very much like the system we tried to develop for Lu'Tamohr, a MUD that was set up by a group of the players and administrators from Dragon Realms. It also resembles some of the ideals from Aarinfel and Azhad: MUDs less reliant on hack'n'slash and more on skills, craft and trade, development through negotiation rather than killing.

I am looking forwards to see if this game really does keep the players around for more than the 7 months which seem to be the average of games like EverQuest. Dragon Realms kept a stable player base for almost 4 years, through a combination of hack'n'slash and role-playing. Aarinfel and Azhad lasted much shorter, Lu'Tamohr was too complicated and ambitious for a bunch of amateurs to get properly off the ground. While there is intellectual challenge in puzzle-type games, there is very little to kick your adrenalin into flushing your veins, and sometimes there's nothing like a good fright!