Retina displays look clear at any resolution -- not just just the default -- thanks to Apple's HiDPI mode. This feature doesn't work natively on non-Retina machines, but a little app called ResolutionTab can help you enable it so you can use lower resolutions without a loss of sharpness.
If you've ever set your display's resolution lower than the native option, you know it looks a little fuzzy. This is frustrating with very pixel-dense displays with higher resolutions (like on the MacBook Airs) if you have less-than-perfect eyesight. Using HiDPI mode will let you lower the resolution without decreasing quality and sharpness. ResolutionTab makes switching through HiDPI modes very simple.
In order to use ResolutionTab, you'll also need to install QuartzDebug -- a free developer tool that requires a free Apple developer account. It's a little tedious, but if you want to use HiDPI mode on your Mac it's time worth taking.
ResolutionTab ($2) [Mac App Store]