I completed Portal a little while back, well, the basic levels anyway. I got my cake. I heard a great song. All is right in the world. I noticed that you can re-play it with “Developer Commentary” enabled. The difference is that throughout the levels there are “speech balloons” that you can click on – when you click on them, various game developers will tell you something about the game or level design. While I have seen and enjoyed similar commentary on DVD movies, it is really neat to have this in a game, to learn about some of the process they go through to develop and refine the game.