Bryan Jones put the iPhone 6 (regular) display under a microscope:
When the iPhone 5 came out, Apple bonded the display to the glass in an effort to get the pixels closer to the surface and Apple has appeared to make the pixels in the 6 even closer still. Some of what we are seeing with the iPhone 6 may be a polarizing filter underneath the glass, but even so, the glass appears thinner and required less focus distance adjustment to get from the surface of the glass to the pixel on another microscope.
I don’t know what that precise distance is in microns between the surface of the glass and the pixels, but it was a shorter distance as judged by rotation of the focus knob in the iPhone 6 vs. the iPhone 5.
What this accomplishes is making the display appear to be higher resolution. The blacks are blacker, contrast is higher and colors are more vibrant, even with the same OS.