Many have already hit on some of the major issues, as a former Dark Sun DM, some of the setting elements that have stayed with me long after closing the campaign (For those who know the setting; I never introduced the events of the Prism Pentad beyond the death of Tyr's king);

-Resource Scarcity. Dark Sun manages to really make you feel like the world has limited resources, from the lack of metal to the risk of heat stroke and sandstorms, players don't feel like shortages "happen to other people"

-Potent PCs vs. Hostile Setting. In 2nd Ed AD&D, you rolled Dark Sun Ability Scores as 5d4 when the default was still 3d6... but you made back up PCs as part of the default character creation, because not just monsters, but the WORLD ITSELF would be a threat to PC longevity.

-Truly alien PC races. From the sterile Mul (half-dwarf) to the roll-for-your-alignment each day Half-giants, to the insectiod Thri-kreen, the new Races of Dark Sun felt far more like alien races and less like humans with prosthetics than any world I had played in before. Even familiar races like Elf (migratory thieves and con men) and halflings (once civilized cannibal savages) took on new, more ominus tones.

-Extreme Variety in limited geography (pre-Prism Pentad). The default Tyr region had a handful of cities each of which presented radically different environs and culture to set stories in.

-Choices that seemed to matter, did your PC take the easy path of Defiling magic or the hard one of Preserver? Did your party have Templars of a Sorcerer King or Elemental Clerics? What you chose about your past and your sources of power had a real impact on how your story would unfold.

Those are just some of my memories of Dark Sun, I'm eager to see it as a 4E product (I run a "generic" 4e game now) but I am cautious as the 2nd Ed attitude of Dark Sun as unforgiving, deadly, and actively uncaring about your PC's fate seems at odds with the core 4e philosophy that the PCs are the destined heroes that avoid the hardships most people must face as they move towards the greatness they deserve.