I've understood that technically the main idea of "retina" is that the physical display pixels no longer map one-to-one with the logical user interface (CSS) pixels. Instead, the retina display appears as a virtual low-resolution screen that is able to utilize higher-resolution images (which would otherwise be scaled down).

So in this sense, I would claim there is a clear technical difference between just higher-resolution screens (more pixels) and retina displays (same pixels but "better looking").

To me this virtualization of pixels seems like a good idea, since the majority of web pages assume pixels have certain DPI range. Operating systems like OSX treat pixels as floats anyway, so if you need subpixel accuracy, it's still possible.