One of the most missed characters is the % sign (as a unit name, '%:0,01), in order to avoid ugly constructs like in the attachment.
Here is, how other units could be defined (percent and per thousand based on a proposal by Davide).
<unit value="0.01" description="[inofficial] percent" dimension="All">
<name lang="eng" text="'％" />
</unit>
<unit value="0.001" description="[inofficial] per thousand" dimension="All">
<name lang="eng" text="'‰" />
</unit>
<unit value="0.000001" description="[inofficial] part per million" dimension="All">
<name lang="eng" text="'ppm" />
</unit>
<unit value="12" description="[inofficial] dozen" dimension="All">
<name lang="eng" text="'dozen" />
<name lang="ger" text="'Dutzend" />
</unit>
<unit value="60" description="[inofficial] sixty" dimension="All">
<name lang="eng" text="'sixty" />
<name lang="ger" text="'Schock" />
</unit>
<unit value="1000000" description="[inofficial] million" dimension="All">
<name lang="eng" text="'million" />
<name lang="ger" text="'Mio" />
</unit>
<unit value="1000000000" description="[inofficial] billion" dimension="All">
<name lang="eng" text="'billion" />
<name lang="ger" text="'Mrd" />
</unit>