Short and to the point always works IMO.
This is how I view the settings anyway.
Forgotten Realms- A fairly basic fantasy setting with a huge following and tons of resources. The basics of the DnD game world hold true, with heaps of new flavor texts. New races and classes (warriors wizards, dark elves, elemental people), lots of classic well known NPCs, lots of support in fiction and video game form.
Eberron- Pulpy fantasy setting in a world recovering from war. Lots of political factions, lots of technology mixed with magic (airships, trains, lots of alchemy), new races (shapechangers, robot soldiers, people possessed by dream demons), high emphasis on Indianan Jones style exploration.
Darksun- A dark, post apocalyptic setting on a blasted desert world. The bad guys have already conquered the world, and the PCs have to make due. Arcane magic is hated, divine classes and the gods don't exist, lots of psionics and mutant creatures, new takes on the races (cannibal halflings, slaver dragonborn, etc), new races (bug people, half dwarves), and lots of new character options.