It's very simple. If you're planning to start a mud, read this book.
Richard Bartle's recent book Designing Virtual Worlds (published by New Riders Group press) is the comprehensive guide to the issues involved in the development of muds (mmporgs, virtual worlds, whatever). In one well-written compendium, you'll find incredibly thought-provoking insights about players, world design (including geography, combat, economics, politics, religion, story, and much more), community management, and social scientific research on what makes muds and their players tick.
Bartle, who together with Trubshaw developed MUD1, the first mud-as-such, has thought about these things for a long time. Although this book is geared primarily at commercial developers of graphical virtual worlds (think Ultima Online, Everquest, etc.) and is illustrated with several examples of design decisions from such worlds, the insights are very applicable to the design of MUSHes, especially RP MUSHes. Bartle does often use examples from textual virtual worlds - in fact, he largely argues that textual worlds are currently much more advanced than graphical ones in several areas.