RetroMud is an extraordinarily diverse and detailed fantasy mud. It is mostly team combat oriented, and while roleplay is encouraged and rewarded, it's not required. This is, in fact, it's biggest drawback: it's one thing to assault a nun or a demon-summoning cultist, it's another thing entirely to do so when accompanied by someone who spouts hitpoint totals.

On the other hand, the areas are vast and immersive, with engaging descriptions and (normally) great attention to detail. I come from a MOO background where each room was expected to have dozens of unique verbs or examinable features, and while RetroMud doesn't normally hit the dozens, it certainly has enough to give it a great feeling of depth. NPCs are plentiful and interesting, though conversation is normally a little thin. Quests abound: roughly 100 are 'official' level quests with a few details up on corkboards at all the pubs in the world, each granting half-cost advancement for a particular level. Counting these there are probably over 200 available, ranging in difficulty from simply asking the right person the right question to requiring dozens of hours of research and exploration and combat.

Possibly best of all, exploration gives experience. Just entering a new room gives xp! Many areas are intricate and mindgame-like, as well, holding secret rooms or intricate backstories for those willing to look at the right clues, search under the right beds, destroy the right weapons, or bribe the right guards. Many players forgo levelling entirely until vast swathes of the six planets have been explored.

In terms of sheer numbers, I've never seen a mud with as many rooms (over 15,000), races (over 60), guilds (19 'primary' ones, such as Templar and Merchant, and hundreds of secondary and tertiary guilds that you can path into), and skills and spells (over 1,000). Especially impressive is that these seem to be quite well balanced, no mean feat as any game designer could tell you.

For new players, an excellent help system a 'mentor' ooc-channel are available.

There are a few other notable pros: there are great themes for each planet (ranging from dinosaur-inhabited jungle to subterranean cave systems), there is the ability to reincarnate and keep most of one's experience, people spend experience on skills and spells instead of using the idiotic and tiresome 'practices' idiom, there is equipment that grants flight or water-breathing (and the worlds are 3D, you can soar through clouds or flit about in the seas), and there is beautifully detailed weather. Ok, fine, sometimes it rains, still...

There are also a few other notable cons: you need to buy a castle (or rent a room in one) to keep equipment across the daily world-cleansing, there's a slight tendancy towards fighting over roleplaying, there's only very restricted player killing, and being in-character doesn't guarantee that others will interact with you in-character.

Still, as examples: Leprechauns can only see in light and ultra-light places, Shadows can only see in dark and ultra-dark places, and Hephastians can see anywhere. To my mind any mud that goes to such detail as to say which brightnesses each race can see in is worth a look at.