Back in the final days of my computer science degree, I was introduced to the game Narbacular Drop; a student-made game made. As a concept it was really novel, and as a computer science student it was amazing to see what a group of students were able to come up with.
I really enjoyed the original Portal, it's very clever in its execution and kept a dark sense of humour throughout the game to keep it interesting. The only problem I found with it was how short the game was and how immediate its finish. Playing through the game last week it took less than an hour from start to finish and I wasn't really trying. But in terms of the concept and what to do with it, Portal is a very remarkable game.
Portal 2 was a worthy sequel, taking what was great about Portal and expanding it. It added enough new features to the environment to make for a variety of puzzles (there's only so much you can do with two portals and the first game had that covered) and gave much more of a story. Possibly the best way to describe it would be to say that Portal was the tech demo and Portal 2 was the proper execution.
But what I think was its stand-out element was its dark, sometimes morbid, humour that was present throughout the story and in a variety of ways. From a psychotic AI with a penchant for personal insults, to an entrepreneur with a reckless disregard for anything (including money or safety) getting in the way of results, there's always something to laugh at in the process.
The game, unlike the original, was long enough. It kept its novelty the whole way through, and only at a few points did it feel like there was no obvious direction to go and I had gotten myself into a dead end. If there was one quibble about this game compared to the original, it's that the ending song was better in the original. The co-op mode is also a welcome addition. When your mistakes kill others, it's comedy. But when their mistakes kill you, it's not very funny at all!
In a year which promises Duke Nukem Forever (finally), Elder Scrolls 5, and possibly Diablo III, Portal 2 has made a very compelling case for Game Of The Year already. If you loved Portal (who didn't?), then Portal 2 is a must buy.