First check out the demo.
Its nifty, but not new, OS X (and Windows) has featured responsive icons for years now (try resizing your icons in OS X). At higher resolutions folder icons are open and 3D, at the small sizes they’re lat and closed. One might argue even the original Mac OS had “responsive” icons based on list views and icon views.
Responsive icons were bound to happen on the web, this is simply an extreme version. There’s been some great discussions about responsive mobile images and different cropping based on screen size. The treatment of responsive has extended in iOS 7 to text, with Textkit. Responsive is a philosophy has been extended to all aspects of design: photo, font, vector image, information hierarchy, structure and even data design.
This demo seems extreme and such drastic icon changes would lead to potentially confusing user-experiences, simply by rotating a tablet and having all the icons change. Also, with the advent of high density displays, less is lost in the mix. I’m not against this, but the demo illustrates a cool-yet-impractical example. I’d love to see an icon set with an OS-like treatment, where the variations of the icons: A little more reservation.
Onward we go :)