- Mailing lists are a (social media) tool for participatory decision making processes, enabling archived discussions with the goal of reaching a concensus. They are used for a very long time in this way in many different setups, from technical standardisation bodies (IETF, W3C), Internet governace (Regional Internet Registries, ICANN), technical communities (NOGs, e.g. NANOG, UKNOF, DENOG...), to community projects such as hackerspaces.
Enabling "tagging" of mailing-lists posts would introduce an improvement in the direction of more modern "social media", such as web-forums (slashdot, reddit) or other platforms where users can "rate" the content (Amazon, FB discussion groups).
"Tagging" is useful feature for several large communities that have many mailing lists (RIPE, NANOG, IETF); but also fo any smaller community (for example, hackerspace) that communicates via mailing lists. Using the karma server would lower the barrier for participation (since it is easier to rate "=1" or "-1" then to emial "mee too", or even more constructive post..).