I just updated to WP 3.9 and my workflow has just crashed.
I am on a Mac, so I don't know how this may apply to PC users but it saves a lot of time to drag selected text into the post area rather than copy and paste - it also frees up my clipboard history for other things.
However, today, I discovered that having updated to WP 3.9, when I try to drag text, I get the 'Drop files to upload' message. If I continue and drop, I get a warning saying that I am leaving a page on which I have entered text (in another field) which is a normal Mac Safari warning.
I am sure that this new feature is immensely useful for some people but for me it is a major nuisance - please give the option to switch it off.