A key usability heuristic termed the "Match between the system and real world", says that software systems should mirror real-world scenarios, nomenclature, and procedures. In the real world, of course, things (especially social) tend to be messy, non-hierarchical, redundant, ambiguous, and often contradictory. A recent post from a designer observed that in his many years of technical support for family members, few could successfully navigate the file system of either PCs or Macs. I, too, have noticed this. People just don't think hierarchically (I have research to back this up, just can't locate it right now). Amazingly, it has taken over 35 years for a software company to completely and successfully shield users from the rigid, hierarchical, and non-intuitive interface known as the "file system" of modern computers. The Xerox PARC Alto and Star systems had, among other numerous innovations (GUI, Ethernet), a user interface that completely hid the file system from users. It introduced the "desktop" metaphor, and that was all users could access - no applications, no file system, just documents. While this approach was nominally copied by Apple and Microsoft, nobody could successfully abandon the file system. Ever since, the user experience of computers has been like an extended version of the ugly, awkward transition of MS-DOS to Windows, where users were sometimes bounced around and forced to open the hood periodically.
Apple's iPhone did this remarkably well on many counts, and seems poised to repeat this success with the iPad. The key is that these aren't explicitly positioned as computers, but computer-enable consumer devices. I'm sure that Apple kept hitting a wall when they tried to bury the file system both technically, and from users - extremely vocal technically adept users shouted down any effort in this area. It used to be Macs were labeled "toy" computers because of the slight layer of abstraction the GUI afforded. They seem to have solved the problem by going full circle, back to the original efforts of the Xerox crew.