You can see image degradation from pixelation long before you can make out individual pixels. I can't really make out individual pixels on my MBA (130 ppi) at one foot, but looking at a MBP Retina at the same distance looks dramatically better. On the MBA, the fuzziness from the heavy anti-aliasing used to hide the pixelation is quite apparent, but on the MBP Retina pixels look like sharp-edged solid shapes.