Dragonstar has been described as "D&D in space." When it came out, I had several groups at my store jump in with both feet. They had a good time.
Dark Sun was a D&D setting back in 2nd edition. You can read more about it at www.athas.org
, the official Dark Sun website. It was set in a sort of post-apocalyptic world, with vast deserts and little metal for weapons or armor. The flavor was sort of Mad Max meets Thundarr the Barbarian.
Characters were high-powered in abilities, started at 3rd level, and all PCs had psionics. However, you could die of thirst in the desert.
In some ways, Dark Sun was my favorite campaign setting. It was designed with adventuring in mind--the character tree concept I mentioned is a perfect example. It lets you switch characters between adventures, so if the party needs a wizard, you just put the druid away for a week and whip out the wizard. Between things like that and the high power, players loved it.
But DMs loved it, too. Most population was bottled into seven city-states, and the rest of the world was wilderness, which means lots of places to drop a dungeon or set of ruins. It was the first setting I've seen where players respect and fear halflings, because halflings would eat you! It was thick with flavor, and it begged epic adventures. Great setting.