Portal 2 already had its work cut out for it way before it was originally announced, because the first game had a cult following grouped in with the Half-Life Orange Box. While Portal did not have a multiplayer mode, many could see that it had the potential to offer an immense teamwork aspect due to the puzzle nature of the game.
The co-operative campaign takes place after the single player campaign and has very loose ties to it, so those who did not play the single player portion of the game wouldn’t be completely lost. In the roles of P-Body and Atlas, players work together throughout the test chambers in order to complete them. Most, if not all, puzzles are completely fruitless if players do not have a second player present.
Completing these puzzles is a lot like science, in that it is exact but based on testing out theories. In Portal 2, your teammate’s life is in your hands. This atmosphere allows for straight-forward puzzle completion, as well as intentional sacrifice of one of the team for the amusement of the other.
Portal 2’s co-operative campaign did not just rehash the single player campaign, it rivaled it for supremacy. It was entirely different, therefore it demanded a playthrough; those that did were not disappointed. If you did not play the co-operative campaign, you missed out on half of the game.