Well, it is just a game.  But for most dedicated players, MUDding is also a lifestyle.  The following sites are dedicated to the study of making good text adventure games, or Interactive Fiction (IF) as they call it.  All of the following sites I have skimmed through, and where I found useful information I've noted it.  Don't get bogged down in the details, but use these resources to guide you stylistically and the sky is the limit.  Happy coding!

The Art of Building
http://www.artofbuilding.net/

An excellent collection of essay/manuals on style and content for coding.  In particular, the Building Guide is full of gems (the individual sections are available in the contents frame on the left).

Crimes Against Mimesis
http://bang.dhs.org/if/library/design/mimesis.html

An eloquent, thorough discussion of how to design an engaging quest.  Useful for any area coder, a must for anyone trying to make a good quest.  My favorite section is that dealing with Puzzles out of Context.  This is my all-time favorite resource.

The Craft of Adventure
http://www.geocities.com/aetus_kane/writing/coa.html

Five essays by Graham Nelson on adventure game design.  Perhaps the most important is the Bill of Player's Rights.

Interactive Fiction on the Web!
http://bang.dhs.org/if/index.html

A site I just found recently, that includes vast discussions of classic IF games.

The Interactive Fiction Page
http://www.cs.cmu.edu/afs/cs.cmu.edu/user/wsr/Web/IF/homepage.html

A site I found looking for another.  Chock full of links to other, more helpful sites.

Interactive Fiction and Adventure Games
http://interactfiction.about.com/games/interactfiction/index.htm

An extensive resource that I highly recommend.  Reorganized since I last used it, but still seems useful.  Includes many tangentially related sites, but the most relevant is the Designing IF section.