A first attempt at sorting 

WITH RECURSIVE
    comment_tree (comment_id, sort_key) AS (
        SELECT comment_id, to_char(score, '0000')
        FROM comment
        WHERE thread_id = 2
        AND parent_comment_id IS NULL

        UNION

        SELECT c.comment_id, ct.sort_key || to_char(score, '0000')
        FROM comment c, comment_tree ct
        WHERE c.parent_comment_id = ct.comment_id
    )
SELECT ct.sort_key || 'Z' as zort_key, c.text
FROM comment_tree ct
JOIN comment c USING (comment_id)
ORDER BY 1 DESC;