Heh, a lot of whine, but no cheese!
Problems that are likely to doom a mud, IMHO:
1) Immortals playing favorites, often involving an SO. When the imm's buddies get perks, and those whom they dislike get big mobs spawning on top of them a bit too often, the game envrionment turns sour pretty quick.
2) The Tinkerer. This is the immortal who spends waaaaaay too much time coding things that are "cool", without really considering (or caring) how they'll affect gameplay. Yeah, my axe of insane-chopping-madness is a bit overpowered, but it's COOL!
3) CommitteeMUD. The opposite problem.... no innovation ever happens because everyone has to "vote" to make any real changes, and nobody ever agrees to DO anything.
4) What's an admin? This is probably the most common problem today. The admins get bored and go off to play WoW or something. The MUD runs on auto-pilot for a while, but eventually people will wander off and find somewhere more responsive.