I always felt that I had a "good" unit, but after comparing with my father's, wow, there's a real difference! Best to see it in pictures...
While the differences may appear subtle on one level, they are not. The fonts are distinctly crisper on the new unit. At magnification, the individual pixels are much more clearly defined. Looked at like this, it is clear that they are being rendered with greater sharpness on the new one. This results in a higher contrast character and therefore, a more legible display.
On important observation is that the large black areas and the background are the same between the two units. They are equal white, equal black, but the E-Ink particles on my original unit are not getting "focussed" as well, resulting in more poorly defined characters.
To me, this proves further variation between the units. Your pixels should be sharply defined. If not, you've got a unit that isn't up to spec.