As I played through single and mult-player I noticed a difference in the mechanics of the puzzles. The puzzles forced people playing together forced both players to rely heavily on each other. The new game elements that were introduced in the single player were presented in ways in the co-op that required both players to depend on the other. The hard light bridge was more difficult to use within the co-op. It was really easy for players to kill each other or themselves if they were not careful with their portals. The gels, especially the orange speed gel, were the most effected by the use of four portals instead of two. One level literally had one player running through their own portals with the help of the gel in order to gain momentum while the other player had to replace one of the running player's portals with one of the non-running players portals to literally shoot them across the level, through crushing spikes, and into a button. Combinations appeared in multiplayer that did not appear in the single players, such as dumping blue gel onto a hard light bridge. With gameplay elements such as these and the possibility with almost endless combinations that could be used in the puzzles, there could be an endless stream of DLC content for the game.