Take everything you love about Portal, then times it by five. This game has so many good aspects to it, it's hard to know where to start.
Sound? Witty dialouge, excellent voice work, a really cool soundtrack, and even something as simple as the sounds that the portal gun makes sound awesome!
Story? Really good. There are some parts to the story where I genuinely felt threatened and isolated.
Graphics? Nothing to complain about.
Although Portal 2 is undoubtedly one of the best games I've played in a while, there are two errors that I find with it:
#1 There are parts in the game where you have to use the "zoom" key to find almost unfindable portable surfaces. I thought this gameplay mechanic was rather silly and annoying.
#2 There are parts in the game where the events that happen don't make much sense, and you wonder why you have to do certain things or why you're in certain places.
Overall this game is one of the best games of our generation, and it's loads of fun. It's good aspects overwhelmingly outweighs my tiny nit-picked errors.