The creative process behind Portal 2
This is a very interesting article where they dive into their entire creative process, including things that they originally planned but scrapped, and how they made decisions. I personally consider Portal the most perfect game of all time (though I prefer the strategy genre so I like Starcraft 2 more), and Portal 2 is right up there with the original, so it is quite interesting to see how they made their decisions.
I would recommend watching this especially for anyone interested in game design, but anyone who appreciates good games will also get a lot out of this. :)