So, as you may be able to tell, I like to have several things going on at once. As well as working on Risus, my Unnamed Fantasy Transhumanism Setting, and running a PbP Planescape game, I'm going to tackle running Dark Sun as a play-by-post.
Dark Sun is, in a word, awesome. A product of TSR's period of trying out new and unusual settings (see Planescape, Spelljammer, Council of Wyrms), Dark Sun games take place on the world of Athas - once a lush, verdant land, but years of powerful "defiling" sorceries abusing the ecosystem have left it a nearly planet-wide desert. The land is split into City-States, each ran by a tyrannic, nigh-immortal Sorcerer-King and his/her Templars (personal police force).
Magic is dangerous - "defilers" can use the life-force of the planet (like the people on it, plants, wildlife etc.) to make their magic stronger. These people are outcasts at worse, and Sorcerer-Kings at best (feel free to redefine "best" here). Most everyone knows this, and arcane magic-users are distrusted and persecuted. As such, psionics (referred to as "The Way") is the main source of power, and is weilded in some small way by everyone on Athas (in game terms, everyone starts off with a Wild Talent - a minor psychic power like telekinesis, telepathy, or being able to smell with your elbow). Divine magics don't exist - the Gods abandoned Athas a long time ago. Religion isn't a massive deal due to this, so most people pay tribute and worship to their local Sorcerer-King.
The rule of the Sorcerer-Kings has led to slavery, and the breeding of races to fill certain niches - like Half-Giants (does what it says on the tin) and the Mul (half-dwarves, who are to a man tough as all Hell). Even the established D&D races are changed - Elves are now desert-dune sprinting, nomadic tribal communities, Dwarves get so focused on one task as their life's work that if they fail and die, they will come back as crazy undead intent on making sure no-one else can do it, and Halflings are terrifying cannibal raiders. Orcs, Gnomes, Goblins and many other races have been wiped out - genocided in the face of the Sorcerer-Kings humanopcentric early rule.
Another big step away from the norm is that there are no dragons (odd for a game called Dungeons and Dragons). Well, there is one - a Sorcerer-King who genocided his own people to gain the arcane might to transform himself into The Dragon of Tyr, pretty much a physical God (and the closest to such you'll find in the setting), and almost killed the sun in the process.
So, all in all - pretty grim.
Starting out for old-school AD&D, and recently ressurected for 4e, it's been a cult favourite amongst a cult gaming community. It takes the emphasis away from collecting loot and gaining power, and puts it into survival on a daily basis - whether you die from stavation, dehydration, or one of the wildlife (Dark Sun is also the only game I've seen which has combat stats for a cactus. Shit grows harcore on Athas), there is one guarantee - you will die. It's just a matter of when.
Will update when I have a better idea who's taking part, and what we'll be doing.