Let me try!
Forgotten Realms: A sprawling, high-fantasy world with dozens of nations described. Evil rules many realms, and the good heroes have died a long time ago or fallen on harder times. Random, magical blue fire gives people deformities or magical powers. Two continents described, one normal (by D&D standards) and one where the Primordials are revered and dragons rule large swaths of territory.
Eberron: High-action and adventure, mixed with magic-as-technology. Adventures often crisscross the globe, thanks to airships. 12 hereditary birthmarks raise 12 lineages to the top of mercantile power. The main continent used to be a single nation, but a 100-year-war turned them into five realms in an uneasy truce since one of them was devastated in a magical cataclysm. If you want to see Indiana Jones riding dinosaurs while through a thick jungle to reach a dragon-made artifact before the undead soldiers of Cobra can unleash it on a New York-like city, this is the setting for you.
Dark Sun: The world is a blasted wasteland, a desert created by the ravages of magic in ages past. What little civilization exists huddles near the city-states ruled by the Sorcerer-Kings, immortal magicians that enslave thousands to toil in the fields, quarries and armies. The gods are absent, and lots of people develop psionic abilities in order to survive. This is D&D in a post-apocalyptic world populated with strange races (like half-giants and mantis-people) or twists on common races (thieving elven runners, cannibal halflings, bald dwarves).