A Gamestop listing, which has since cut important information out of the description of the title, revealed a nice chunk of details about the sequel.
The first and probably biggest news is that Portal 2 will feature two player Co-Op. The story and test chambers for the multiplayer will be entirely separate from the single player campaign.
The single player mode will have new characters in the story, while gameplay wise there will be more test chambers than before and new puzzle elements to push the players wits.
The sequel is going to take place in Aperture Science Labs and everyones’ favorite computer GLaDOS will be returning.
Gamestop lists Portal 2 to be due out this Holiday season.
(Original Description Source: Kotaku)