During the Diablo Lore panel at BlizzCon 2011, three new areas were shown from Act II. All three of these areas are playable and play an important part in telling the story as we progress through this Act.
Caldeum - Caldeum was the jewel of the East, and the trading center for all of Sanctuary. However, within the city there was turmoil, due to the Zakarum, rich merchants, and Wizards all vying for power over the city. Hakan eventually took rule and united the three groups to create the most prosperous city; however, his death left a son to young to rule the city. The Iron Wolves, lead by Asheara (from Diablo II), were hired to protect him until he was old enough to rule the city. The young ruler removed them and replaced them with his own Caldeum guards. Asheara is unsure of why he made this decision.
Dahlgur Oasis - This small paradise was the birth of Caldeum. A mysterious man rose from the waste and showed the original settlers this source of water which was used to help begin Caldeum. However, the story is said that this mysterious man disappeared back into the waste after showing them.
The Archives of Zultun Kulle - Zultun Kulle was one of the original members of the Horadrim who helped capture the Prime Evils the first time. However, he eventually became corrupt and evil. He was so strong that they could not kill him they cut him in half. They removed his head and buried it underground surrounded by magical runes. His body has sent into another dimension.
BlizzCon 2009 Demo
In the BlizzCon 2009 Demo, player starts in an area called Sundered Pass, and is tasked by the woman called Asheara (at this point, it is unknown if this is the same NPC from Diablo II) to reach the town called Alcarnus, which has apparently been desecrated by the Cultists. The player then sets off through several areas, of which some are the Stinging Winds and the Canyon Rim Mines. Some new monsters were revealed, like the Desert Dervish and the Desert Wasp. At the end of the Monk Reveal Trailer, the hero fought against a Morlu Caster, but this enemy did not appear in the Demo.