Like others, I too upgraded my WordPress Installations to 3.5 with a hope to get a new experience of Media Uploads. On some of the installtions it worked fine, but on Biharprabha News, it started causing issues.
The Add Media button on the Top of Text Editor didnot render the popup when it was clicked. The same was case with Set Featured Image, which too rendered a popup earlier. If you too face the same problem, here is a quick solution to this.
Add following lines to wp-config.php file.
define(‘CONCATENATE_SCRIPTS’, false );
Make sure you add this just before these lines
In my case the issue was resolved after adding this. If you face the problem even after trying this, do let me know by commenting below.
For setting Featured Image, it is possible to Open the Link in a New Tab and set the Featured Image, but for adding any image to the content there is no workaround