Dark Sun gave a lot in terms of character power but demanded a lot in terms of actually influencing the world in which the characters operated. It was challenging for both players and DMs to adjust their imaginations to such a radically different setting.
That being said, we have a lot to look forward to. DS was always the best setting for for people who like psionics because it was an integral part of the world. New races like the Mul and the Thri Kreen were inspired, and the DS take on Halflings was awesome. The classes were very well crafted in that they tied directly in to the setting in a serious way: there wasn't much difference between a Greyhawk mage or one from the Realms, but the Defiler was loaded with neat gameplay and role-playing opportunities. I hope they give Gladiators their due--given the balance of character class powers in 4E I think they'll be better this time around.
Anyone who thinks they might want to run a Dark Sun game would do well to get their hands on a copy of Freedom (Adventure DS1). It was a great story to kick off a new DS campaign and will make a nice conversion once we have the new rules in hand.