Im new here but what the OP highlights is exactly what I have been aiming to do , in the sense that I have seen all graphics has to offer. (I have played lots of MMOS and gotten into almost every genre)
I am not experienced enough with muds yet though and I think it in some sense makes me applicable to your query. I have gotten into 1 mud, the three kingdoms. The thing I liked about that was quests had a storyline, good NPC interaction and it was not just hack/slash with minimum coded NPC interactions(For someone coming from MMOS , diverse NPC interactions with standard quests and quests that link to a global plot are essential) Also good ways of talking to all character types , npc and monster alike(Yes NPC mentioned again
, its that important!)
Also fitting into to the criteria of "People that dont generally play muds but comming from MMOS" well I think quests have to involve examining room descriptions and finding things out, also very very important is a logical connection of rooms so its easy to map. You see I thought materia magica would have been for me but I was told to look on their website for maps, now heres the thing, I tried to map it with a mapping tool but sometimes in the same area you would just beam illogically to other places for instance you come north into a shop , go out the east door of a shop and end up in some street way to the south.( I think mapping is part of the fun, as YOU the player are exploring and finding stuff yourself) . The three kingdoms would be great for many MMO players and it nearly got it right but some of the quests were too hard in that unless you had mudded for years and were armed with a multitude of command words/not so obvious words gotten from 5 to 10 years of mudding , well you could get 3/4 through a quest and never get any further and it was highly illeagal to talk about how to do a quest(I like a challenge as much as the next person and hate using cheats/walkthroughs but the inexperianced do need a slight clue or helping hand from time to time and thats what stopped me playing that mud)
I hope you dont mind me relating my own situation its just I like this post, and I think more MMO players would be attracted to muds if quests were well thought out and had diverse storys but were attainable and people didnt delight in someone struggling with one part of a quest that for months they couldnt get and had been through every single word in the room descriptions but couldnt get help even though they were putting major effort in.
So
Attainable quests
Balanced help for quests - not handing it on a plate but giving clues what to try if a person has been stuck for days/weeks
Deep story
Really good NPC interaction
Perhaps PK but only for characters in the end game band of levels in which they can take over and run towns
A mud where any group of players can create a guild , not just people who know the mud admins
Logical mapping
Leveling
Talents
Good gear
Players can buy in game property(Im not talking any pay to play or alike schemes here)
A well run political element
Elements of resource gathering and crafting(Players can perhaps now and again make rare 1 off gadgets)
Enchanting
Player Marketplace as well as NPC shops However NOT RL money based because it ties in with a post I seen a moderator on this forum made which I 100% agree with. I.e Mr Uber who plays 24/7 or maybe even uses a borderline bot devalues resources and everything else so that the majority of non hardcore players cant make any amount of cash at all of their goods(This is a problem in many graphical MMOS)
Ways of customising a character so that its not just a case of do a build way X,Y or Z or you have a bad build and will struggle the rest of the game
Good Newbiie zone with quests that perhaps have multiple ways of doing things
I know its alot to expect but muds have way more scope than graphics games if you think about it, theres many many more things I could have added but if someone were to come up with something like the above I think absolute hordes of people would play , mudder and non mudder alike. I know I will like MUDS but I havent found one that fits yet. I actually think a mud that kinda goes down the interactive fiction route would attract many MMO players