Might be the best Valve game ever made! This game is not to be missed!
First of all I want to comment on the story, which is very good. You play as Chell and have to make it through the Aperture Science facility once more. It appears that GLaDOS was not truly defeated in the end of the first game, for you will accidently wake her up again, with help from your robotic companion named Wheatley, a funny little prick! There are some very interesting turns and twists throughout the game which will fill the story and make it more interesting, this I think Valve have done splendidly. So the story gets an outstanding 10/10 from me.
The source engine still holds up very well and the graphics look amazing, and it fits the game perfect. I'll give it 9/10 and not 10/10 because it doesn't really break new ground, however it's very nice nevertheless.
I don't think I've ever heard such a good sound in a game ever like I have in Portal 2, the voice acting and all the other sounds are so clear that it's insane. 10/10
And last but not least the gameplay! For those of you who played Portal you all know how the portal gun works, but the interesting new feature they've added to Portal 2 is the gels! There are two different ones, a blue and a orange one. I will not spoil exactly what kind of effect they will have on the gameplay, because some of you who read this may have not played this masterpiece yet. But I assure you they make the game extra fun to play! So to sum it up the gameplay easily gets 10/10 from me.
9.5/10 Thank you Valve, now go make Half-Life 3!!!