I’ve been using a Retina MacBook Pro for one week, only as a secondary computer,2and I’ve already changed my font, redesigned my narrow layout’s header, and conditionally replaced an image with text. I’ve noticed that fonts, especially, respond extremely differently on the Retina screen: many of my old, non-Retina choices simply didn’t look good, and many fonts and metrics that were previously poor for screen use can be used nicely on Retina screens.
This so true. Retina and HiDPI displays are a small percentage of the market right now but their increasing in numbers very fast. In addition, designing for such displays requires more than just doubling the resolution of the images displayed on the website. Typography is definitely one of the affected areas.