Since upgrading to WordPress 3.5, I had trouble on one of my sites where I couldn’t add links, media or edit in text mode. I found a fix by adding this in the wp-config.php (above ‘stop editing here’):
define('CONCATENATE_SCRIPTS', false);
From the WordPress Codex: http://codex.wordpress.org/Editing_wp-config.php#Disable_Javascript_Concatenation