Valve's panel at the GDC yesterday was titled "Creating a Sequel to a Game That Doesn't Need One". Can you guess what it was about? TOO LATE! It covered the conception, design, and release of Portal 2, and it was bloody fantastic. All sorts of wonderful aspects of the game's early design was included in the presentation, including the fact that at one point, the game had no Chell, no GladOS, and no Portals! Valve's lead writers Chet Faliszek and Erik Wolpaw ran the audience through a ton of stuff that drastically altered during the development cycle of the game, and showed us a bunch of awesome video clips of once-integrated mechanics and environments that unfortunately didn't make the cut.
Initially, the player awakes on the middle of beautiful, serene tropical island, only to have the roots of its artificiality revealed in a gorgeous deconstruction sequence. Glass barriers raise of the ground to trap the player, palm trees get sucked down beneath the floor, the surrounding "ocean" drains away, and the warm sunlight is harshly cut off, revealing the player is, in fact, in the middle of a barren warehouse. This totally different opening sequence was attributed to a version of the game that lacked basically everything about it that made it a Portal game in the first place, called F-Stop. In F-Stop Portal 2, Cave Johnson was a Southern Billionaire and GlaDOS was replaced with a personality sphere named BETTY, who followed Cave around in a glass container on wheels, spouting an endless stream of legalese at the player, explaining how Aperture Science was in no way liable for the ridiculous danger the player was about to experience.
Further versions of the game had a completely different Cooperative story line, which took place after the single-player campaign, instead of sort-of during. In this version of the story, GlaDOS realizes Peabody and Atlas aren't actually providing any useful testing data since there's no human observing the experiments. This poses a problem as the Heisenberg Uncertainty Principle states that the outcome of, well, anything, is only known once it's been observed! Since all GlaDOS has are robots driven by AI and not sentient humans, she can't use any of the information she's taking from their test chamber runs. The ultimate goal of the robot's work ended up in helping GlaDOS make sense of a comic strip they'd found on the abandoned bulletin boards deep within the Aperture facility.
The only other truly significant aspect of the game that was mentioned as being cut was the ability to carry Wheatley around and drop him at will. The player could even smash him up against the game's geometry, and Wheatley would scream in pain and plead for mercy. Valve initially designed a staircase in one particular area simply so that players would drop Wheatley on it during playtests. There was a video segment of this, as well. Bloody hilarious.
I'm sure a ton of other stuff was mentioned in this way-too-short panel, but with any luck this presentation will see the light of Youtube in the near future, and you can see a whole of bunch of the videos that were shown for yourself. In fact, check Youtube now. Go, I'm not even kidding. There were at least a thousand people in the audience. Everyone had cameras.