I love how they have taken something that looks absolutely modern by today's standards, and they have made it look ancient by having it overrun with plantlife. Little visual clues like this that suggest an extended history to the Aperture Science facilities make them fascinating places to simply explore. While there is obviously a puzzle in each testing chamber, the bigger puzzle is why are labs all here and what the hell happened to this place?
LIMBO is another example of a game with beautiful minimalist graphics. The chiaroscuro netherworld you walk through in LIMBO makes it seem like you are lviing in a charcoal drawn work of art. While I wasn't a huge fan of LIMBO's learning-through-dying-repeatedly gameplay, I did love all of the contrast and shadow of the artstyle.
I do not have anything to add, but I do want to say that this is a fantastic thread. Please keep up the great work. I also agree that it would be great if you could organize a gallery event. (Not that I could make it, but I would be excited to read about it.)