I'm in the process of helping with the design of our mud website. Since I am doing most of the content, I'm interested to know what your opinion is, on what should be on a mud website.

Sanctuary of the Forsaken
mifx3.ipoh.org 4000

You should have something that at least suggests the theme of your mud.  You should have or point to instructions telling interested visitors how to connect to your game.

I have found there is a wide variety of things people put on their websites, so there are no hard rules to follow.

The best advice I can give you is to look at many, many other game's websites.  The pay ones, as well as the free ones. See what others are doing.  Decide which aspects of their sites you like, and which you don't.  

Then reconcile what you'd like to see in your own website, versus what you can actually accomplish given the time and resources you are able to put in.

If staff permits make all your helpfiles available online. This is an invaluable asset with people perusing the website first. They can get a feel for the game's mechanics if they are even just Rom or common. People can read up on helpfiles about IC things usually, and other stuff.

Then the common stuff.
Story of the realm.
Maybe player submitted things.
a discussion board link if you have one.
Rules link

there you go.