It's tough to describe how awesome it is without spoiling anything.
Perhaps the greatest aspect of Portal 2 is the writing. Forget Metal Gear Solid with its endless, expository dialogue. Forget Final Fantasy with character archetypes and melodrama overload. How amazing is the writing of Portal 2? About halfway through the game I actually started caring for an AI robot that once tried to kill me. This is in-between all the laugh-out-loud dialogue. I couldn't believe it but Portal 2 managed to outwit even the outstanding original.
Oh right, the gameplay. The basic elements of the original are all still here, but there are some new concepts that really make things interesting, like gels with various properties, reflective lasers, light bridges, etc. The areas are much, much bigger this time around, there's plenty more variety, and man are some of the puzzles cunning. Expect to spend over half an hour on a couple of the gel-based labyrinths, assuming you like to get things done without a guide.
I'd like to recommend this game to virtually anyone who considers themselves a gamer, but especially those who can appreciate smart writing and gameplay that goes far beyond button mashing or shooting things. Portal 2, while not as groundbreaking as the original, is the kind of game you wish would never end.