The desktop metaphor is meant to covey to the user that the files and folders on their computer are similar to the files an folders on their desk. In addition, the work that the user does on their desk is similar to how they work on a file when it is open in an application. However, when introduced computers were very large (and often on the desk) and monitors were using CRT technology. Even notebooks took up a monitor sized footprint on the desk. As a result the computer was not just another work area, it actually replaced your physical desk with a virtual one.
In the last decade, CRTs were replaced by flat screen monitors, often more then one. Because of their smaller size they leave useable space on the desk and many can even be directly mounted to a wall (if the desk is against a wall). Therefore, and especially with multiple large monitors, your virtual desktop is less like a desktop and more like an interactive wall above the desk with your physical desktop available for traditional, non-virtual uses.
As a result the concept of the desktop seems to be becoming less relatable then it once was and new form factors like the tablet are making this detachment even worse. The virtual world seems to be expanding into the physical world as people can now walk around with windows that before were limited to their computer screen. The advent of the cloud and networking has also changed the ways people store and move files, with the expectation that they are simply everywhere.
What is needed is a successor to the desktop metaphor that can capture the new ways of working in this physical/virtual world but still be applicable when applied to the contemporary desktop.
