The original Portal was just a side project done by a couple developers and thrown in with Valve's Orange Box collection. Despite its rather low-key release, the innovative game play, quirky humor, and stellar voice acting made it a huge hit. The game was released as a stand-alone on Xbox Live as Portal: Still Alive with additional puzzles, and fans still wanted more. Valve originally announced the coming of a sequel in an innovative way, by releasing a title update to the PC version of Portal that embedded codes in the sound files of in-game radios and altered the game's ending so the protagonist's escape ended with her being dragged back by an unseen hand.
Portal 2 was released as a stand-alone game, developed by an entire team with all new puzzles, new elements, and the introduction of a two-player co-op mode. All these new elements come together and make the sequel a HUGE SUCCESS.
Note: while I will try to avoid details of the story and end-game, some of the items discussed here may be considered spoilers. I avoided media and reviews of the game before I played, and I enjoyed the reveals as I hit them in-game. Decide for yourself if you want to read on.
The single-player story mode re-introduces the original game's player character, Chell, who is being held in a hotel room-like chamber in suspended animation, periodically awakened for "exercise" and "culture" sessions (walking around the room once and staring at a picture on the wall for a few seconds). The final time, she is awakened by Wheatley — a personality core brilliantly voice-acted by British comedian Stephen Merchant — who tells Chell of a catastrophic failure in Aperture Science and offers to drive Chell's room to safety. As the facility crumbles around you, Wheatley drives your room on railed tracks to the beginning of the real game — the same "relaxation chamber" where you started the original Portal. As you leave the chamber, you go through several familiar puzzle rooms (now crumbling and overgrown with vegetation), re-acquiring the portal gun and re-learning the basic mechanics. Eventually, Wheatley unwittingly leads you to re-awakening GLaDOS, and the more advanced testing begins.
You go through GLaDOS's puzzles until Wheatley finds you an escape route, which eventually leads you far below ground to the long-buried chambers of Aperture Science's early days, guided by recorded messages from Aperture's first CEO, Cave Johnson. There, you learn of the long-forgotten experimental substances — a red-orange propulsion gel that increases your running speed, a blue repulsion gel that lets you bounce to new heights, and a white conversion gel that lets you apply portals to surfaces that normally wouldn't accept them.
The story and voice acting is phenomenal. I often found myself standing still and waiting to see all the dialog that GLaDOS, Wheatley, and Johnson had to say. The puzzles are interesting, although I found them to be a touch on the easy side. The puzzle elements were always right where you needed them, so while it sometimes was a trick figuring out how to put them together, it was rare to be so completely stuck where I didn't know what to do. I never once felt hampered by the controls or camera angles, though, which is a rare joy for a puzzle/platformer. I managed to finish the whole campaign inside of a week (although it was helped by a full sick day off of work, where lying on the couch playing Portal was the only thing I could really do). The biggest down-side, however, is, because it's a puzzle game, once you figure the puzzles out, there's little reason to repeat the experience. In a second playthrough, with nothing new to figure out, you just go through the motions.
In co-op, you and a partner play as two robots created by GLaDOS to solve special cooperative tests (the "Cooperative Testing Initiative"). Each robot is given a portal gun that shoots two distinctly-colored portals (red and yellow for one, light blue and deep purple for the other). The portals are linked only to each other (e.g., you can't go through a purple portal and come out a yellow one), so even with up to four portals in play at any given time, it's a lot more manageable than it sounds. Communication is vitally important in solving puzzles, and it is made much easier with quick tools for "pointing" to locations (so you don't spend 10 minutes trying to explain "which wall on the left" you're trying to point out), starting countdown timers (which are not subject to the slight lag of voice chat), and viewing your partner's point-of-view.
Again, the puzzles were pretty straight-forward. Knowing what to do was never an issue, even if the how took a bit of time figuring it out.
The real Achilles' heel of multiplayer stems from the biggest problem in single player, that it's just not as fun the second time around. In multiplayer, it's rare that you'll find someone who's at the exact same point as you (except when it was first released, and no one had played it yet). Which means you end up with one of two situations: one player is just leading the other through the motions, or they're trying to stay silent while letting the other player figure it out. It's a less-than-ideal situation. Figuring out a new puzzle together is the most fun, but it happens so infrequently, and those moments are only going to get more rare as the game is played more.
DLC has been announced for the game, which should increase its usable life, but only temporarily. Also, while Valve has a history of releasing DLC cheaply or free for their PC games, there is a very real possibility it will be free for other platforms and be forced for cost on the Xbox 360, like what happened for Left 4 Dead content last fall. Perhaps having the content on the PlayStation 3 at Valve's pricing will pressure Microsoft to ease up on their cost mandates, but we will see.