I was comparing the features of some Lumias and one of the display "features" for the Lumia 920 and 820 is "RGB Stripe" - what makes this so special (say, in comparison to the 800 that doesn't list it as a feature)
It refers to the pixel layout on the screen. On a RGB stripe display the red, green and blue pixels are the same size and have the same count. In comparison, a PenTile RGBG uses a smaller green pixel (since the human eye is most sensitive to green), so the screen ends up having less pixels than a RGB strip display of the same resolution and is therefore cheaper.
The RGB Stripe layout gives better colours and a sharper image when compared to something like a PenTile layout. Some fonts look very jagged on a PenTile layout but look very crisp on a RGB strip layout--I also find them to have a strange green hue.
The Lumia 800 has a PenTile display but due to the small screen size it isn't as noticeable. The larger 900 has a RGB stripe.
You can clearly see the difference between PenTile layout (left) and RGB stripe (right) in the below image.