There's a brief interview with Valve's Erik Wolpaw talking with the lead writer on Portal 2 about the puzzle/platformer sequel. When asked about any features they cut from the game, he explains that they removed some nauseous wall walking gel: "At one point we had a new Gel that allowed players to walk up the side of walls. While this added a new twist to the game, the effect was so disorientating that it made people nauseas [sic]. While working on a first-person game you always have to be aware of that issue so we removed that Gel. We also made changes to frame rate and movement that will help anyone play Portal 2 even if they do experience a feeling of nauseas [sic] while playing other first-person games." Thanks Joystiq. Also, this trailer shows a level that was cut from the game because it would be too difficult, featuring more delicate timing than most levels in the game. Thanks Reddit.