Having the same issue on multiple sites. On some of the sites (based on theme) the featured image link in posts/pages is completely gone. In other themes the featured image link is there, but when I click on it, it openes the media library full screen. I can select the photo I want to add and click on "set featured image" but then it doesn't give me the opportunity to go back to my original post. If I hit 'back' in my browser it will go back to the post but without adding the featured image.
This is on multiple sites across multiple hosting platforms and multiple browsers. I've disabled all plugins and it still hasn't fixed the issue no matter what the plugins are.
Here's hoping a 3.5.2 update is coming soon???!!!