From a web-app perspective, this will change significantly the life of webmasters and website content creators. When Apple introduced the retina display in its iPad, iOs developers had to create and maintain two sets of images, one for devices with the smaller resolution and one for devices with the increased resolution. Users complained about the sudden increase in apps disk usage despite using an old device, and some ran out of disk space while updating their apps to the latest version.
With the introduction of the Retina display for MacBook Pro, every website owner will have to realize, sooner or later, that he can either adapt or be conquered: viewing a traditional site in 2880 x 1800 will probably be like watching an HDTV trailer on a 1990 TV set. The race is on: fixed, non-fluid, 960px-like layouts are dead, and different versions of graphical icons and logos (at a minimum) will be a must in the post-Retina world.
Despite their high price, the new laptops are expected to conquer significant market share in the near future: they have lower noise due to asymmetrical cooling fan blades, an incredible thinness of just 1.8 centimeters and 2.05 kilograms in weight.