Remember, the Retina display isn't about resolution, it's about DPI (Dots Per Inch). A higher DPI means you have a higher resolution, but a higher resolution doesn't necessarily mean you have a higher DPI (2880×1800 on a 15" screen is about 220 DPI, whereas 2880×1800 on a 27" screen has a much smaller DPI).
The operating system and your application have to take into account the DPI in order to give you anything that looks good. If you go on programming your game assuming there are 96 pixels per inch (which is what used to be the "normal" measurement several years ago and now many programs assume that to be true, despite better, higher DPI screens being made now), your text, images, etc. are going to look much smaller than you planned on when I draw them on my 15" 2880×1800 display. This is one reason why on OS X, you have to use the Cocoa and Quartz 2D frameworks to get your program to be rendered in high DPI (because otherwise when your program communicates with the operating system, your program is going to be assuming a low DPI screen and the OS has to accommodate for that).
When drawing text (and images) to the screen, pixels is a pretty bad way to measure the size of things, because 100 pixels on my Retina screen is going to be a lot smaller than 100 pixels on another "average" monitor. When doing graphics, you still have to deal with pixels, but if you don't think about how big those pixels are to the user (that is, how many pixels are in an inch?), things you draw may be too big or too small.