Act I (Diablo III)

The first Act of Diablo III brings us back to Tristram, where an unknown object fell from the skies and struck the Tristram Cathedral. The Official Web Site states that this 'harbinger of doom' will awake ancient evils and call for heroes of Sanctuary to defend their world. Not many areas of the Act were revealed, but those that were revealed are showing what the entirety of the Act could look like.

Tristram Wood
The player starts in Tristram Woods, where he is tasked to follow the road to the old Tristram Cathedral and investigate what happened there. The first NPC you talk to is Captain Rumford, who then tells you what happened in Tristram. The player then proceeds to follow the path to Tristram, or New Tristram as it is known now, and eventually to the Tristram Cathedral.

Tristram Cathedral
Once inside the Cathedral, the player finds himself in a level called Forgotten Tombs (the name is subject to change). While proceeding deeper into the dungeon, the player witnesses the destruction that the unknown object left behind. The holes that it left were shining with blue or turquoise color. On the last level, the player encountered an old enemy from the past, the Skeleton King.

Leoric Highlands
Although this area was not playable in the BlizzCon 2008 Demo, it was shown in the Gameplay Trailer at the WWI 2008. The level was filled with various monsters, like the Khazra, Scavenger, and the Gnarled Walker, but none left such an impact as the Siegebreaker Assault Beast. The main thing that was remarkable about this boss was his size, far larger than any of the bosses from Diablo II.

Update
The above information is pre-release. While the beast listings are accurate, the Siegebreaker Assault Beast has been moved to being a mini-boss in the middle of Act III. The final boss of Act I is the horrific, murdering demon The Butcher.