I have dualboot and so always compare ubuntu, gnome and win, just the GUI!!!
If you know win7 and then compare it with unity, you will see that there are not may differences. So, I don't think that unity is something entirely new. It is just compiz like before with a panel and a dock, which is the same like win7 taskbar, just with a global menu.
Why? Quicklists = Jumplists
both combine starter and tasks (or grouped windows)
the grid in unity and gnome 3 is the aero-something in win
the start menu of win7 startmenu can filter recent documents with mouse over the related app. Unity has nothing like this.
the Lenses in Unity are realy different and will have some good features in the future.
gnome3 is realy a entirely new, next generation desktop. Unity just made its way into the present.
Maybe Unity likes to make it easy for windows user to start in the Linux world while gnome3 is too different.