There are many different opinions on exactly what makes a good area.

• The Project Web-Archive (long) deals with aspects of creating a whole world.
• The MUD Building FAQ by Deori has some good advice for newer builders. Personally, I disagree with his take on perfectionists though. ;)
• The WoTmud IV Building Guide has some good general tips as well as tips that are specific to their mud.
• Crimes Against Mimesis contains a long, but very good article on staying in context.