[Adrian Schneider]

Try this... untested, as I have no data to test this on.

SELECT user.userid
     , user.username
     , counts.comments
  FROM ( SELECT userid
              , COUNT(*) AS comments
           FROM reputation
          WHERE reason IS NOT NULL
         GROUP
             BY userid ) AS counts
INNER
  JOIN user
    ON user.userid = counts.userid
ORDER
    BY counts.comments DESC
 LIMIT 10;