The design is that there really isn't a "desktop mode". the desktop is a feature/surface of the system, it's not something you switch your whole computer into.
While perhaps technically true, as a user you really don't notice any of that. When you log in to Windows 8, it automatically shows the Start screen. While this is Metro, all you have to do is start any desktop application, and the desktop appears.
This is no different in Windows 7. After logging in, surely you would start any application, either by opening the Start menu, or by clicking a application pinned to the task bar, or by double-clicking a shortcut on the desktop. You can put anything onto the Windows 8 Start screen (just right click a file or directory and select Pin to Start), so it's functionally identical -- just with bigger buttons.
And if you really don't like it, you can always move the Desktop tile to the top left, so you can enter the desktop by pressing Enter.
