Oh GLaDOS, how i love you so...Let me review the ways...
The gameplay is simple, you make portals using the left and right mouse button. You jump using space and you move with the WASD kets.
Nothing to it. If you haven't played Portal 1, stop reading this right NOW, and go play it. You cannot play Portal 2 with full enjoyment without playing the first Portal. Trust me, it is worth it.
If you HAVE played Portal 1, you know what happened. You defeated GLaDOS (*sniff*) and landed in the parking lot of Aperture Science. You are then dragged back into the facility and put into a sort of "stasis" room. Here, you wait and sleep, wait and sleep, and the sucks you in abrubtly. As soon as you meet Wheatly (A blue eyed personality core with a West English accent) you will fall in love with the game.
The voice acting is superb, top notch, first class, AMAZING and epictastic! You will laugh out loud at Wheatlet's antics and you will feel nervous as you traverse the huge expanse of Aperture Labs.
The graphics are based on the same Source engine as the first game, albeit with a few modifications that make the game look better than ever. The areas look gorgeous and the designs over the years is noticeble.
This is not a game to be missed! You MUST play it if you have played the first one :D
PS: GLaDOS is awesome and she owns all of you 0_o!!!