The first thing that I did when I logged in to Xaos back in '98 was get lost. I'm not a good navigator in the best of times, and the place that I had logged into was twisted and strange. It was a winding garden full of giant bugs, vegetables, and centaurs (my chosen race). I wandered for a while before finally saying "I give up, how do I get out of the centaur village?" on the out-of-character channel. I think that it was an immortal named Albach that answered me, telling me the way out, which it turns out didn't exist. In the end he had to get a way out to the rest of the world built, because someone had forgotten it attach one, and I was the first person to be in there.
Originally I had started out as a centaur warrior. Before the first day was over, I had decided that actually wanted to be a mage, so shortly after starting I made my second character, a gnomish mage and then gnomish illusionist named Xz. I've been playing Xz ever since, and still enjoy playing him, for the stories, roleplaying, enemies and friends that I made and make with him (Ural, my centaur warrior, died a well-scripted death years later).
The first person I met "face to face" was the white dragon enchanter, Kestra, (self-proclaimed keeper of love, hate, and the phoenix gate) who as I remember came down from his mage's tower over the human town and agreed to help me out instead of eating me, and made me a fully enchanted set of armor. Later he would become a hermit and a curmudgeon as a result of establishing himself as the person for newbies to ask for powerful sets of enchanted armor.
Then I met Alexander, the human knight from a foreign land (who would eventually become one of my character's enemies), who showed me around the then relatively small world.
And Aelioane, the timid healer, haplessly prone to being attacked by everything under the sun, from gods to men (Ael made it fun).
And Genghis, the brazen leader of the trollish horde, who would eventually kick my character from his clan in a round of racial purification.
And Shosuro, who ironically served as the horseman "Pestilence" (despite being female) for the nightmare king.
And Albach, the nightmare king, who my character would eventually swear fealty to and spread the word of strife and treachery as the horseman "Famine".
Those were fun days.
Needless to say, in the 9 years since '97, Xaos has grown. There are automatic maps to help navigation while outside of the cities, there are probably 20 more racial starting locations, maybe 8 more classes, more new areas than I could count. Oh, and in the meantime I've become an imm myself, and built areas, run quests, and before too much longer I'll have started out a new race.
Off the top of my head, my favorite parts of the code of Xaos would probably have to be: shapechanging spells, creating illusions, combat attacks (like chain lightning) that take skill to use right, mirror image (which makes a duplicate of its caster), raising undead, the arachnia clockwork race...
and my favorite history/roleplaying of Xaos would be: the playing out the role of a horseman of the nightmare king, the attacks of crystalline beings (called Slaraecian) from another plane, dealing with the politics of the leadership of the mages' tritower, the skirmishes and attacks of the trollish horde, laying plans with allies to advance the purpose of my clan...
In time between starting out on Xaos and now I have checked out a lot of other muds. None of them stuck, with the exception of the Discworld mud (with its beautifully executed humor and tie in to one of my favorite author's worlds) and Aarinfel, an extremely roleplaying-intense mud. Unfortunately, Aarinfel died, and as for Discworld, while I still play it from time to time for the amusement at the concepts, I find the code of the place is often excessively complicated. 8 directions, 555555 channels, players yelling ooc things like "+7 stat ring for 15 crowns and 7 brass!", mobs that look like pcs, they're all things that I can do without. Xaos' code is pretty straightforward and pure, straight from the hand of the same coder for the past 9 years. That's not to say that there isn't complexity in the game, there is, in its areas, mobs, objects, classes, races, and players. Not in its communication, or movement, or looking around, or getting objects. 'cause in the end that's not what I'm looking for. For me, what's important is interacting with other players, and getting good roleplaying of one kind or another with them, and that's where Xaos really shines. Isn't that what's important to you?