Well, most gadgets have backlit screens; a light source illuminates what's on the screen so that you can see what's on it. Unfortunately, looking at these displays for long periods of time can strain the eyes. The Kindle uses e-ink, which is basically a grayscale screen that is not illuminated. More easy on the eyes, but you need a light source to see what's on the Kindle's display.
Regardless, I'll have to try out the DX to see if it provides a decent reading experience. The larger format means I can see more content at a time, but it might be more cumbersome to carry around.