This thread is about El Capitan, so I tried all 3 of your examples on El Capitan, and all functioned as you expected, but not as you stated from your experience. Apparently those functions are not missing in El Cap, at any rate.
That was: Dragging a file to a folder in the dock causes the folder to spring open from the dock.
Dragging an image file onto the Mail app opens Mail, with a document window showing the image.
And, dragging a file across the Finder icon, showed the all folders on screen, allowing you to simply move the file wherever you like.
So, I wouldn't give Apple feedback on areas that seem to function in the way that I remember.
I'm not sure what you may have changed in your system, but those various finder actions don't seem to have changed dramatically, and do what I expect.