Certainly the number of subscribers, and their growth rate (month to month) is usually of interest; similarly number of monthly messages posted, notable Subjects discussed, abstract of any personal thanks you may have received (anonymized), perhaps what you see as the added "value" of having the discussion list.

You might think of this effort as forming the basis of some milestone PR.  E.g., here is a short 'blurb from May of '02 that was the result (in part) of the list stats.

http://www.nasfaa.org/publications/2002/ty10thfinaid051002.html?Print=Yes