Running a mostly hack'n'slash Mud myself, I go by my own preferences. And judging from those I'd say that diversity is important - and the free choice.
I get bored like heck in a pure RP Mud, where there are no monsters to kill, or you have to have an IC reason to go on a killing spree. Bashing monsters is really good as stress relief at times. And watching people emit 4+ line emote all day long can be tireing.
On the other hand, I get equally bored, if killing monsters is all there is to do. And especially if it is the only way to advance in a mud. Killing can get awfully monotonous after a while.
So we try to set our Mud up in layers. There is a surface layer, where a zone can be played just like some hunting grounds. The important thing on that level is to make the monsters diversified and at least semi-intelligent. They should not just stand there and wait to get killed. They should use different fighting tactics, pursue you when you flee, shield or block attacks, load helpers, or have other defensive tactics to resort to. They should have skills varying with their level and tier, so that newbie mobs are easy, but high level mobs have lots of nasty tricks up their sleeve.
The next level is the 'hidden' parts of the zones, the doors and portals and sercret containers that you need to search a bit for to find, and where some of the goodies are hidden. This needs at least some attention from the players, and the ones that turn brief mode on usually miss the best parts. (You should never FORCE players to read all the room descs, but the ones that do should always be rewarded in some way).
The third level is the Quest option - instead of killing the mobs you talk to them, find out their needs and try to tend to them. For this you get rewarded too, in different ways, and it also gives an extra incentive to explore the mud. To find all the mobs, objects and tricks needed for a successful quest you may have to travel over half the world.
About communication channels, I think they should be simple, and that there should be several choices. I don't really care if it is not logical to be able to 'tell <playername> <message>' even if the player is in another room far off. I once played a semi-RP mud, where the 'tell' problem was handled with the sentence: 'You send a dispatch to <player.name> with the message <message>'. It may have been logical, but in the long run the extra spam just became irritating. Keep it as short, sweet and simple as possible!
Finally, there should be other choices. If you just want h'n's - fine, but those that want to RP should be able to do so, and there should be options added to facilitate and reward RP, like meeting places, channels, RP groups etc. Those who prefer making a living from a trade or profession instead of killing stuff, should be able to advance their char in that way too. There should also be some games within the game, for those who like a bit of hazard, and if you are lucky that too should be a way to advance.
And ideally players should be able to get as much info they like from their prompts and score commands - but they should also be able to turn that info off when they don't want to see it - just as they can turn the channles off that they don't want to listen too, or turn off the battlespam messages, if they don't need to see the detailed info about how each hit lands. I hate spam myself, so all ways to reduce it are good to me.
Diversity is the key - and the free choice. If you want it easy, you get it easy. If you want it complicated, there should be a number of extra options to go for. And the learning curve should be as steep or as slow as you choose.