Well, as someone who has taught lowly end-users, I can see where they might have problems. (Remember, there are lots if users who still don't understand the concept of folders/subfolders...)
For example, users will forget where they put stuff on that huge desktop. They'll spend lots of time scrolling hither and yon trying to find an app or file. What navigation/find features will this desktop have for those users?
Does it have to be 3D? Why not just a 2D scrollable desktop?
Nauseating!!
There's a freeware Macintosh app (the name of which I cannot remember at this moment) that was a 2D-scroller. You could make the desktop as big as you wanted and use the arrow keys to slew 1/2-screen or whole-screen in any direction you wanted. One of the function keys zoomed you out to see the 'whole world' and you could click on a spot to zoom back in on it. Ran in OS9, never bothered to see if it ran in OSX. Pretty neat though!