@Ian2: The 'not simple' bit (IMHO) is that the start screen will force desktop users to act like tablet users -- and that's quite a disconnect.
If they just added a 'don't use metro start screen' option for the desktop then I think a lot of desktop users will be a lot more happy about upgrading. MS do seem to be expecting tablet users to be the majority.
But without the Start Screen, I don't think Windows 8 offers enough on the surface to cause users to want to upgrade. After using Windows 8 on my main machine since the beta came out, I have to admit that I'm used to the start screen now and am just waiting for some better metro apps. The Win+I and Win+C shortcuts needs to be more visible though; the hot corners with the mouse is something I almost never use.