Roleplaying enforced. I'll occasionally stray to the odd PK mud, enjoy Godwars and Wotmud.org in particular, but I mud for the roleplaying. If there are global channels, mass mindless PK, or out-of-place zones, like smurf-village just north of the medieval city, you can pretty much count me out.

I also expect different takes on your generic fantasy races/archetypes. For example, Armageddon's elves aren't the kind and just elves most of us are familiar with. Instead they're quick-witted thieves and tribal raiders. Armageddon's dwarves aren't gruff, bearded fighters but are, instead, hairless, utterly focused beings. Halflings? No Frodo, Sam or Tasslehoff's at Armageddon. Instead, you get cannibalistic warriors who jealously guard their forest homes.

I also expect any mud I play for long to have a helpful and courteous staff, a decent-sized playerbase, an account system to discourage griefers, and finally, a unique crafting system.

So, Armageddon is pretty much the perfect place for me. It combines all of my interests, mud-wise, into a beautiful experience. Mmm.

-WP

No matter what type of place it is, you should always expect fun out of a mud, that's for certain. Different people have different ideas as to what constitutes fun for them. I started out with a hack&slash type place, and that was fun. I moved to a roleplay encouraged place, and that's been fun. Sometimes it can be tough to keep that rolling, no matter where you are.

You probably either need some sort of goal that you want to attain in mind (getting to a certain level, playing your character through a certain situation, etc.), or at least have new options always availble. That's one thing I expect out of a mud if I'm going to play--there needs to be something to grab, and then hold my interest.

It's also nice to have a community feeling to the place. I don't like to be disconnected from the people that I am interacting with, I enjoy it a lot more when I can get to know them a bit aside from any mob/player killing, or roleplay or such. Being able to come together and have a nice big family type of feeling is really great.
A big part of that is probably staff, and newbie friendliness/accessibility, though the players themselves need to take up the cause too.

I'm not so much on board in any sort of debate between 'commercial' or not, roleplay or not, etc. I've been able to find good stuff in several different settings.