Materia Magica was the first MUD that I played for longer than a week. I took two characters to around level 60, so while I cannot comment on the more advanced features, I feel I have a better good feel for the mud.
First, despite the reason I stopped playing (mentioned later), this is one of the best MUDs I've played. Seriously. Great coding combined with attention to detail. I didn't spot any glaring grammatical mistakes or spelling errors. The writing was terse, yet informative. A model for any other MUD.
Secondly, the only reason I kept playing so long was the quality of the "puzzles". An example would be a new tower added, whereby you had to rescue a princess from a dragon (If memory serves me correctly). Unlike all the other MUDs I've played, where you're effectively going to storm into the castle, kill everything and get the treasure, this quest required logical puzzle solving abilities, and as a result was the most satisfying quest I've done. Anyone can level and kill things, not everyone can solve a decent mental challenge.
I'll skip over all the other well done things (travel, other dungeon areas, exceptionally friendly player base), and skip to the real reason I decided to write a review: Quests result in every character being identical.
Because all the numbers are visible, and because if you just keep doing quests, you can eventually upgrade your stats all the way, every character in the game feels identical. Everyone played the same set of classes, with the same skills, all maxed out. Eventually I simply gave up because it seemed characters were too similar for my liking. Skills should be much harder to get, and it shouldn't be easily possible for a class to get a high score in all the available skills with ease.
If MM ever changed the questing system so that it offered no help in progressing with skills and stats, I would be back in this game quick as a flash, but until then, I feel it lacks.