If Blizzard's Diablo 3 FAQ page is still acting wonky, or if you're too impatient to read through everything here are the highlights:
Return to Sanctuary, the same world as the previous 2 games. The game takes place 20 years after Diablo 2.
There are 5 character classes in the game. We know two so far: a new Witch Doctor class, and a completely redone Barbarian class. Male and female options for all classes.
Havok-powered physics
Building on previous random environments, Blizzard promises "a host of new ways to create random scripted events throughout the game."
Upgraded Battle.net
Deckard Cain returns, but is noted as the "first victim" of the new horrors
Plan is to release the game "simultaneously in as many regions as possible, and to localize the game in several languages." The game will ship for Mac and Windows simultaneously.