The reason many are worried is that Windows 8 is bringing with it the Metro experience found on Windows Phones, and similar experiences found on the Xbox 360. There will be an underlying "standard" UI, but it wil be a minimalistic approach. A look that is earily very similar to Google's look in some pictures. I am not against the minimalist approach, but only when done right. On top of that, Metro is a touch-centric UI. Critics and beta testers have already been stating that it doesn't translate very well to a keyboard/mouse interface to well. It is a tad clumsy.
So, in regards to what I have said, why could Microsoft be the death of the PC? Windows Xp is finally on the way out the door. Microsoft officially no longer supports it. Vista is quickly right behind it, as Microsoft seems to wash their hands of the OS, even though I personally never thought it as bad as everyone else made it out to be. Windows 7 will be supported for quite some time, as the platform was fairly solid, even though I am not a big fan of the interface. Windows 8 will be the defacto OS installed on nearly ALL devices released within the next several months.
Many users are jumping on board the Tablet craze. And smartphones have quickly replaced many peoples means of interacting and interfacing with information, social networks and communicating with friends. Android and iPhone have also proven themselves as viable gaming platforms for many, getting away from the restrictive desktop gaming experience.
What I am expecting is Microsoft is wanting to usher in a touch world with the desktop PC. HP and Dell both have some impressive desktop devices that use Windows 7 and touchscreen interfaces. Both work rather well. However, these devices are a minority. A majority of PC users don't have these devices, or monitors that perform as such. The other restrictive aspect is the "All-in-One" design of these devices. PC Gamers will refrain from jumping on board initially as they want the ability to upgrade graphics from time to time to maintain the edge in graphics and physics in their gaming that an "All-in-One" does not allow.
Keep in mind that Windows 8 is still in beta. A few things could still be missing. A few things may not be fully implemented just yet. In due time, we will see how it all plays out.