To activate the debug mode of WordPress, add the following PHP code in wp-config.php:

define('WP_DEBUG', true);

This SQL query remove all revisions of posts having the topic, reply and attachment type (tested on WordPress 3.1.x):

DELETE child.*
FROM wp_posts AS child
LEFT JOIN wp_posts AS parent ON parent.ID = child.post_parent
WHERE child.post_type = "revision" AND parent.post_type IN ("topic", "reply", "attachment");

Count the number of posts, pages, revisions and comments produced by each user:

SELECT COUNT(u.ID) AS content_per_user, user_nicename, u.ID AS user_id
FROM wp_users AS u
LEFT JOIN wp_posts AS p ON p.post_author = u.ID
LEFT JOIN wp_comments AS c ON c.user_id = u.ID
GROUP BY u.ID;