Oltman at El33tonline writes:
"Portal 2 is much more of a game than Portal 1 was. It has a much better thought out plot, the story has a few surprises and the lore of the Portal universe is greatly expanded on. It also has co-op multiplayer for the first time. Where Portal 1 was deemed insignificant enough to be bundled in the Orange box with a few Half-Life expansions and Team Fortress 2, it has grown into a full-fledged game with a DVD cover all of its own. It is now also around 12 to 15 hours to complete the entire singleplayer game compared to the original’s 3 to 4 hours. Expect your second play through to be much shorter as you know all the solutions by then."