Quick Note: Portal 2 is amazing. The single player campaign is the right style of challenge and hilarity that will make you want to play again and again. The co op campaign adds an extra layer of awesomness to the game and is highly recommended.
Portal was such a unique game. With smart puzzles, a unique gameplay mechanic and a villain who controls everything around you, it was an interesting experiment that led to a cult hit. With the release of it's sequel, everything has been taken to eleven and it's astounding.
Portal 2 has two story lines, one for single player and one for local or online two player co op. The single player story line picks up an undetermined amount of time after the original, with Chell being awoken from her Relaxation Chamber by a personality core named Wheatley. The game follows the two as Chell is reunited with her Portal gun, unintentionally resurrects GLaDOS and the attempted escape from her test chambers. It's just as exciting and witty as the first game, with a broader scope, encompassing a lot of Aperture Science's past as well as Chell's and GLaDOS. This is a game that if you follow the trophy/achievement list, a lot is unveiled in the background, per Valve's smooth sense of style.
The co op story follows two test robots, Atlas and P-body, who were created by GLaDOS to replace any human test subjects that are no longer available. Though the story line is not nearly as obvious or straightforward as the single player, the humor is just as strong as GLaDOS plays the two players against one another.
The graphics are fantastic, with the sparse, simple style from the first game long gone and replaced with a degraded, over grown style that makes Chell's escape seem so much more desperate. The game even starts with some of the original test chambers, showing off how fair the test facility has gone without GLaDOS. The music works well, adding the electronic/techno flair to resemble the science theme of the game.
The gameplay is almost exactly like the original, with the player utilizing two connecting portals to solve puzzles by pressing buttons, placing boxes and using physics to reach the end of each test chamber. In order to add variety, the team at Valve has added some more mechanics in each level. Three types of gel are added into the mix. Repulsion Gel, which is blue, allows the what it touches to jump high into the air. Acceleration Gel, which is orange, accelerates what ever comes into contact with it. Finally, Conversion Gel, which is white, allows the player to place portals on any surface coated in it. Combined each gel adds it's own special layer, but combined the puzzles become awesome brain teasers that are completely rewarding to solve.
Aside from the gels, there are also the Light Bridges and Aerial Faith Plate which increase movement capabilities across the levels. Light Bridges are solid bridges that are made of light and therefore can be transferred through the portals to other areas of the levels. Aerial Faith Plates are plates that launch whatever touches them into the air in an arch. Again, though these mechanics seem simple, putting them into good use and mixing with the gels and Portal's unique game mechanic makes for some interesting puzzles.
The pacing of the game is great, allowing the player (both old and new) to get into the swing of things without ever feeling like he/she is being babied. Each puzzle progressively gets more difficult, but not in obvious ways. Sometimes the complexity will change, and some times it's just a way of thinking. All the puzzles are fair and most of them are not frustrating, with the exception of maybe a few of the later puzzles.
So with out ruining the game, I'll wrap things up here. The game is a fantastic work of art and completely enjoyable. If you skipped out on the first game, that's ok because this one covers everything. The game play is solid, the humor and story are amazing, and the co op is surprisingly enjoyable. I highly recommend buying it, owning it and playing it over and over again.
10 out of 10
Official Game Website: http://www.thinkwithportals.com/about.php
IGN Review: http://www.ign.com/games/portal-2/xbox-360-64330
Wiki Page: http://en.wikipedia.org/wiki/Portal_2#Design
Gamefaqs Page: http://www.gamefaqs.com/xbox360/991101-portal-2/faqs