So… I upgraded to WordPress 3.5 today and unfortunately became a victim of the now widespread problem of dynamic content (javascript/jquery, etc) handling. To make it simple, the post editor was gone and no content was showing when trying to add new or edit existing posts and pages.
To everyone experiencing the same problem, you can temporarily fix it by adding the following to the top of your wp-config.php file:
//temp fix for WP 3.5 not loading dynamic menus (jquery etc)
define('SCRIPT_DEBUG', true);
Hopefully the WordPress team will come up with an official fix to this issue soon.
UPDATE: It seems that the problem was caused by the well-known BulletProof Security plugin and it has been fixed by the developer. Good job 