I think it’s fair to say that probably every game is made, at least in part, by committee. The results can be fairly obvious; it’s easy to see when some executive has made the decision to pursue a Call of Duty or a Gears of War.
Valve, by contrast, produces perhaps the purest product available on the current market; the games where it’s clear that most decisions have been made in the interest of making the best videogame possible.
Portal 2 exemplifies this condition; it is pure game, straight through, with each facet specifically calibrated to produce what, undoubtedly, becomes an instant classic the moment the disc enters the tray.
The sequel returns the player to the life of Chell, unknown Aperture test subject, as she awakes hundreds of years after the events of the original game, once again trapped in the Aperture facility. If this change of circumstance is somewhat mystifying, then I recommend reviewing the updated ending to Portal (dispensed via Valve-patch), or better yet, reading the online comic bridging the events of the two games.