Often when converting images from true-color to
8-bit, there is the problem of color fidelity.
Dithering should take care of it, but there are
some pathological cases. One of the canonical ones
is when a "pure" color, especially white, is
thoroughly used in the image. Often GIMP, instead
of keeping the white "pure" (that is, #ffffff),
will change it into a degraded white. This is very
annoying because pure white is easily recognizable
(e.g. on a Web page) from even a very slightly
degraded white.

There should be either an algorithmic improvement,
or an option, to keep the white "pure" when
switching to 8-bit. It would be very useful.

An example is here :
http://rezo.net/~antoine/install1.png
Using GIMP 1.3.19, I cannot find a way to convert
to 8-bit (indexed) while keeping a pure white.

Conversion to indexed mode (255 or less colours)
is important for WWW, especially for size reasons.