As can be seen by some of the comments left on the site, a couple of frequently requested features cannot be added to the List Authors widget because they are not supported by the underlying wp_list_authors template tag. The WordPress code for wp_list_authors needs to be changed to enable these new features.
Feature 1: Allow an upper limit to the number of authors listed. Some WordPress sites have hundreds or even thousands of contributors. wp_list_authors currently would list all of them and, worse, execute an SQL statement for each author. The proposed fix could be a non-negative integer option named “count_limit.”
Feature 2: Allow sorting of the author list. This could be an option named “sort_by” with the values “name” and “post_count.”
Today I submitted a patch to WordPress Trac including both of the above features. It also changes the code to use a JOIN statement to get the author post count instead of running an SQL statement for every author. This was a necessity to prevent inconsistencies when applying the LIMIT and ORDER BY. One of the core developers is reviewing it now and, if I’m lucky, the change will be included in the next major release (version 3.0). If that happens, you can be sure I will be updating the List Authors widget to make use of the new options.
A less frequent request but one I’ve seen is for an “exclude” option with the ability to filter out certain categories of authors. There is a separate ticket for this and I might consider submitting a patch for this next.