As a professional game developer, I've always loved how well Valve designs all their user experiences. They highlight this very well in the developer commentari

As a professional game developer, I've always loved how well Valve designs all their user experiences. They highlight this very well in the developer commentaries which are included in all their games. To me, those commentaries are one of the best parts of any Valve game, despite often sounding like an infomercial about how great Valve is. Now, the portal thing isn't all that sensational. The room which is a level selection menu has been done a ton of times. Nintendo loves it. Super Mario 64 is one of the earliest examples I can think of and Nintendo Land even presents the different levels as attractions in a theme park which are accessed via futuristic looking gates (portals?). Valve simply did the same thing with the portals being more like those in Valve's own hit franchise aptly named "Portal", but as standalone 3D objects, rather than flat ones projected onto a flat surface.