If I had years to design the perfect mud, I'd make it just like 
AderonMUD. The design is so well integrated and makes so much sense. 
A river winds throughout the mud, connecting the major points of 
interest, but there are also portals for quicker jumps. An extensive 
area help system is provided to tell you which areas are good for your
level and where to find them. There are 5 classes (fighter/mage/
arcanist/rogue/psionicist), each of which specializes in 1 of the 5 
major stats (str/wis/int/dex/con), which is a stroke of genius.  As 
you advance in each class you can train that particular stat up to a 
certain limit. You'll switch from class to class, learning all the 
skills & spells of each class as you advance. After you've obtained 
50 levels in all 5 classes, you'll be eligible to move up to Avatar, 
where you'll be able to train your stats way up, giving you incredible
power, which you'll need to face the really high-level mobs.  
You haven't lived until you've traveled to the Temple of 8 True Gods, 
stopped at a trainer to practice, and seen: 

          Gerim's High Priest SHREDS you with her hit!  
          You wish your wounds would stop BLEEDING so much!  

There's so much more.  Your character will be able to morph into an 
animal shape, giving you incredible new battle skills, like going for 
your opponent's hamstring, and when that attack succeeds, ripping out 
his throat. You can also enhance your weapons and armor by using 
skills, spells, and embedding jewels.  You can have your own house,
join a clan, and go on quests. If you reach the upper Avatar levels, 
you may become immortalized by having equipment and weapons named 
after you. I've seen lots of muds, but I've never seen one this good.
Come see for yourself.

AderonMUD is in the process of upgrading from CircleMUD 3.0 to the new
tbaMUD 3.51, and yet still retaining the last 10 years-worth of
changes!  We are becoming bigger and better than ever!