I'm the developer of a screensaver that draws over the desktop, so I have looked into this since upgrading to Windows 8.
The problem is that the screensaver runs on its own separate desktop (the login screen is a separate desktop too), so it doesn't have access to the current user's desktop to copy the background or display a semi-transparent window over it. This isn't a
case of adding some code to work around it either, because the screensaver desktop doesn't have permission to access the other desktops.
In my own screensaver, this means it can't display images over the desktop, it can't open an explorer window*, and changing the desktop background doesn't work properly either.
I'm sure it has been done for improved security, but it's still very annoying.
* It actually can open an explorer window - but it opens on the screensaver desktop, which is utterly useless.