The photographs below compare the same icons and text on a MacBook Pro Retina display versus the display on an 11" MacBook Air. See the original blog post here. (Note that these images are 220 PPI, so you might need to give them a few seconds to load.)
Text on an 11" MacBook Air.
Text on a MBP Retina. Much sharper.
The Mail.app icon on a standard display.
The Mail.app icon on a retina display. If your monitor is clean and you look really closely, you can see a few dead pixels.
A close-up of the Mail.app icon on a standard display.
A close-up of the Mail.app icon on a retina display.
The menu bar on a retina display. Notice how the updated icons look great and those that haven't been updated yet look like crap. Unfortunately, this is what most of the internet looks like (with the exception of text, which looks great).
The dreaded ghosting issue. You can also see several pixels misbehaving in this photo (top center).
The one curious exception to text looking almost universally better on the retina display is the Twitter application. For some reason, the text looks as bad as the scaled-up profile pictures.
A stack of Macs to give you an idea of relative size. From bottom to top: standard 15" MBP, 15" MBP Retina, 11" MacBook Air, and a 3rd generation iPad.