[GeoffR]

I think smoothing is basically a process of adding shading pixels to the glyphs to give an illusion of curves, or an illusion of a line being drawn part way between pixel rows. Sharpness is just the absence of smoothing. You can think of the sharpness slider as being a smoothness slider, but with maximum smoothness at the left.

Smoothing is most useful on low DPI screens or for small font sizes. It is less noticable on high DPI screens like the AuraHD's. Even on my Glo I can't really see much difference between maximum and minimum sharpness at the font sizes I normally use for reading unless I take a screenshot and zoom in so I can see the individual pixels.

Weight and sharpness are related in a way. To make a sharp font heavier the lines have to increase in pixel units, whereas a smoothed font can be made heavier by rearranging the shading pixels that were added for smoothng. So I think for large/smooth fonts the weight slider will show a more gradual change than for small/sharp fonts.

Because the sharpness setting affects how the the weight slider works and vice versa, in practice you need to adjust weight and sharpness together to get the best effect. In firmware 3.2.0 the weight and sharpness sliders have been combined into one.

To return to the factory default settings you can edit the Kobo eReader.conf file and delete all the readingFontSharpness and readingFontWeight lines.