Valve have been including commentaries with their games since Half-Life 2: The Lost Coast, all of which I've sat through and been genuinely interested in. They talk about the ideas behind plot developments and gameplay mechanics and how everything works from world portals to HDR water. The most recent valve game, Portal 2 has brilliant commentary as it tells exactly what motives the developers were going for with specific parts of the game, and shows that the technical stuff behind most of what you'd take for granted is shockingly complex.