I would love to hear your feedback.

>>   (save-restriction
>>     (widen)
>
> Please add a comment explaining why widening should be used here.

Widening is not necessary any more.

>>               do (let ((buffer-read-only))
>>                    (put-text-property (point) end
>>                                       'invisible my-dired-hide-details-mode))
>
> Better bind inhibit-read-only to t.  Or better yet, use
> with-silent-modifications (but move it outside the loop).
>
> Also, rather than set the invisible property to nil or t, better set
> it to another symbol (e.g. `dired-details'), whose meaning is then
> controlled by add-to-invisibility-spec.

Right.  Obviously this simplifies the implementation.

>> (defadvice dired-insert-set-properties
>>   (after my-add-hide-props (beg end) activate)
>
> Obviously, this would have to be turned into a patch, and since it's
> not small, it would need to be moved to its own function (which would
> be called from dired-insert-set-properties).

Check the new patch.  Altogether 7 new lines are added to
dired-insert-set-properties.

>> (defadvice find-dired (after my-fix-move-process-mark-to-arg activate)
>>   (move-marker (process-mark (get-buffer-process (current-buffer)))
>>             (save-excursion
>>               (goto-char (point-min))
>>               (forward-line 1)
>>               (point))))
>
> How is that related to dired-details?

This is not necessary any more.

My former implementation hid every non-file line that
dired-insert-set-properties is called upon.  Vanilla dired calls
dired-insert-set-properties on every line except the directory
headerline so my-dired-hide-details-mode hid the information line.

    total used in directory RMS available VI

Initially find-dired inserts the directory headerline, newline and the
find arguments and set the process mark is set to 1.

In the process filter of find find-dired evals

    (dired-insert-set-properties (process-mark proc)
                                 (1+ (point)))
    (move-marker (process-mark proc) (1+ (point)))

with (point) being right before the end of the last complete line added
by find.

That is dired-insert-set-properties is called on the first and second
line so my former code added the invisible property to the directory
headerline as well.  This is why I added that advice - just move the
process mark to the second line so dired-insert-set-properties is not
called on the directory headerline.

I removed the hiding of non-file lines.  Hiding full lines via text
properties might cause confusion when it comes to interactive line
movement.  There are no changed to find-dired.el now.

        Christopher