Portal 2 keeps the original game's style, but with some changes since it takes place several hundred years after the original game. At first, you will find your old testing grounds, but overrun by plants. Later, you will work your way through new tests, but considering the state of the facilities, even the new locations are in shambles, but that's okay because you will find yourself in much darker and drearier places before the game's Single Player Mode is over.
One of the major aspects of Portal that really sold the original game for me was the dialogue. While there was pretty much just you and GLaDOS, with the occasional lonely-sounding turret thrown in for good measure, the game would have been far less memorable without them. In fact, the game would have felt like just another puzzle title instead of the psychological head-game that it was.
For the most part, that feeling continues in Portal 2. You have a couple more voices to keep you company this time around though. One such character is Wheatley (Stephen Merchant), who can get a bit annoying at times, especially since he was apparently designed to be the most moronic A.I. ever developed. Another is the voice of Aperture Science's founder, Cave Johnson (J.K. Simmons), but I don't want to talk too much about his role in the game as that would reveal a bit too much about the Single Player story. Suffice it to say that the added characters do a lot to add content to the game and allow for a few changes to keep you playing.