I’ve just spent 2 hours trying to figure out why I couldn’t move my widgets in the admin panel (presentation / widgets) since the upgrade of wordpress 3.5. Apparently the new version of jquery (1.8.3) causes problems with some jquery related plugins.
As usual it’s always advised to desactive all your plugin and to reactivate them one by one, till the moment you see which one was involved in the issue. In my case it was the plugin called “recent post enhanced”. So for those who have it installed and facing the same issues (impossible to drag and drop widgets), just desactive it.
Hope it will help someone around.