DartMUD is unique among other MUDs for a number of reasons. Towards the top of the list is that the game game features a permadeath system: if your character is killed then (barring exceptional circumstances) you will find yourself rolling a new character.
The game is entirely free of a class system, all characters are able to pursue whichever skills they desire. The skill system is especially interesting for a number of reasons. In the skill system, as with most things in DartMUD numbers are hidden from the player wherever possible. Your skills and attributes are cast in english phrases, and each phrase represents a varying number of actual attribute points. As such it is impossible to tell how skilled your character is (except in terms of how capable you find him at performing a certain action) or how strong your character is (except in terms of how capable he is at lifting a known unit of weight). Skill points cannot be purchased -- instead they much be learned by performing actions relevant to the skill -- and the actions need to be of a difficulty appropriate to your character's current skill. Too easy and you will not learn -- too hard, and you will not learn.
Complimenting the skill system is the "concentration" system. Each action your character performs costs some measure of your character's concentration, which is regenerated quickly over time. Ascending a cliff face might require a large degree of concentration whereas picking up an item would require less. As your concentration wanes any skill check becomes more difficult -- and at the extreme lack of concentration would lead to unconciousness.
The magical system is not seperate from the skill/concentration system, but an integral part of it for characters who learn magic (the same goes for the combat system -- both are equally rich, and containg delightful nuances). Your character's mana pool is represented by the colour of his aura -- with a dim red at one end of the spectrum, and a scintillating myriad of colours at the other. Learning spells requires practicing casting them -- at a proportion of their nominal casting cost -- adding additional energy makes the spell easier -- and casting at less than the normal amount is harder.
DartMUD offers an amazingly diverse system of crafts -- from woodcarving to blacksmithing, mining to herbalism. The tree of items gatherable and craftable is astonishingly deep, and inspired later crafting systems in online environments such as Ultima Online.
The single most interesting thing about DartMUD is the playerbase. Unlike many other muds where guild admission is handled by some automated system DartMUD's guilds and houses are entirely player-run -- as is the bulk of the economy, and most everything else which happens in the game.
To attempt to alleviate my bias, I will try to comment on the negative as well. The descriptions of the areas in DartMUD are often bland. The threat (and promise!) of permanent character death holds in check any idea of exploration for all but the most powerful or foolhardy. Command syntax is often painful and seemingly counter to what the room and or help text would suggest. Part of the background of the world includes extensive race wars -- and some characters will expect you to be familiar with the details of them -- but those details are not shown in the helpfiles and introductory information.
Overall DartMUD ( telnet://dartmud.com:2525 ) is as enjoyable a roleplaying experience as you could hope to find online -- with the added bonus that it is free.
