I usually hate when people discuss Valve games as they seem to hold Valve on this unattainable pedestal of perfection in game design. I never thought the Half Life games were the pinnacle of game design that some people apparently infer, but i have to give credit where credit is due for Portal 2.
Portal 2 takes everything that was wonderful in the first game and adds a nice additional layer to it that adds interest without adding undue complexity. Even a new player can hop into this game at almost any point and get through the levels. The worlds overall are much larger and more interesting in this game and the back story that Valve has crafted for Apeture Science is incredibly well done. You get a hint of what this place was all about without it being completely spoon fed to you.
Voice acting is incredibly well done with the new characters of Cave Johnson and Wheatley. I had some genuinely smiling moments in the game.
Everything about the world and the story is really well delivered. Everything is there for the user and the story elements are laid out with very good subtlety that when you begin to understand what happened and is going on in the story, the game makes you feel like you just unlocked some grand mystery. Even the final moments of the game have some wonderful little nods and hints as to what is really happening.
One of my favorite games of this console generation.