Next round using #reduce:

    def special_selector
      [
          [:$query,          selector],
          [:$readPreference, read_pref],
          [:$orderby,        sort],
          [:$hint,           hint],
          [:$comment,        comment],
          [:$snapshot,       snapshot],
          [:$maxScan,        max_scan],
          [:$showDiskLoc,    show_disk_loc]
      ].reduce({}) do |hash, pair|
        key, value = pair
        hash[key] = value if value
        hash
      end
    end

Improvements welcome.