From a thread on HIG:

Seth Nickell wrote:
> The current feedback from this operation is poor and seems to suggest
> that the operation will allow you to position windows (when in fact it
> does not). You're correct in observing that this is a source of
> confusion and poor usability. Direct manipulation in the form of a move
> with no feedback that "something else will happen" (for example, folders
> highlight when you drop over them to indicate that the icon will not
> move to that position on the screen, it will drop into the folder) means
> "move this object on the screen to the place where I released the
> mouse". This needs to be fixed...

When I read this, I thought it meant that the workspace to be dropped into
should be highlighted in some fashion.  I'm not sure if that's what Seth
meant, but it sounded like a good enough idea to me that I'm entering it
into bugzilla.

This is (hopefully) a solution for two problems: it isn't obvious which
workspaces will get a window when the window is dropped; and it isn't
obvious that the workspace-switcher doesn't allow precise positioning of
windows (just workspace-grain positioning).