Portal 2's combination of slippery, bouncy, and portal-enabling gel was more than enough to keep our brains confounded during our second rendezvous at Aperture Science, but apparently yet another gel was planned for the game's many puzzles. Valve lead writer Erik Wolpaw reveals as much to EA.com in a recent interview, detailing "a new Gel that allowed players to walk up the side of walls."
During the game's testing, Wolpaw says, Valve concluded that the gel's use was so "disorientating that it made people nauseas [sic]." Inadvertently imparting nausea is a constant concern when designing a first-person game, he explains.
As a result, Valve "made changes to frame rate and movement that will help anyone play Portal 2, even if they do experience a feeling of nausea while playing other first-person games." As evidenced by that feeling in the bottom of our stomach, however, it's clear that Valve did little to mitigate the sheer terror felt when plummeting hundreds of feet.