I have no idea why. I was doing some routine maintenence on my site when I finally decided to investigate WHY none of my authors could add featured images to any of their posts. I deactivated ALL of my plugins and found the culprit: Achievements for WordPress.
Just thought I'd let you know.
I am running WordPress 3.7.1. I know your turn around is fairly quick and all of my users are big fans of the Achievements functionality.
I do have a roundabout way of adding featured images, but it's a bit of a stretch for non-tech folk to understand.