As it’s easy to see, Exclusion Mode really turned out to be a rather wild beast to tame.
I got the feeling that the most of its natural usage lies in the field of channel masking, where its black-gray-white oriented functionality and the three "Rexx Rules" mentioned above would provide at least somewhat expected results.

Other than that, I managed to produce some non-acid colors was when I:
1. Extracted the highlights (Ctrl/Cmd + Alt/Option + ~)
2. Copied them to a separate level (Ctrl/Cmd + J)
3. Inverted them (Ctrl/Cmd + I), and finally
4. Changed the mode to Exclusion.

To see the difference between the result and the original more clearly I also made a mask with a few windows, which I stroke with white and set in Difference mode to guarantee the edges to be always visible :-)

Another, somewhat similar, yet different image was produced when I:
1. Selected highlights again (Ctrl/Cmd + Alt/Option + ~)
2. Inverted the selection (Ctrl/Cmd + Shift + I)
3. Copied the resulting “shadows” to the new layer (Ctrl/Cmd + J), and
4. Set the mode to Exclusion.

In both cases the resulting image was less saturated than the original.

Exclusive Difference

As we predicted from the very beginning, the two modes are hard to call intuitive.

Difference mode ended up being simpler of the two, essentially doing what it name implies.
Immediate application can include “image proofing”, black text becoming transparent and white text becoming inverted, and, as such, always visible.

Exclusion mode with it’s highly nonlinear formula and heavy gravitation towards pure gray can be either used for quick-and-dirty “gimme some weird colors” projects, or, on a much more sophisticated level, for separating shadows, highlights and mid-tones in the channel-based operations.