People HATE changes, until they've been around long enough not to be "change" anymore. It's simply not time for metro yet, that will come much later. That said, I think it's impossible to site windows 8 as "the" smoking gun. It's the #1 thing people are going to complain about because it's just that different.
What's killing pcs? For the general public, tablet saturation, large phones, slowing pace of software complexity. You can run office from a web browser now. You can do 90% of what you want conveniently from a tablet without burning your lap. your 3 year old hard drives don't fail like they did back in the days, Consoles are still in full force. Where is the pc's place? The number of things you need a pc for, (specifically a new pc) is shrinking quickly every day. Sure /.ers sue pc's and can use all the power they can wrap their hands around, but grandma is just as well off reading emails on her ipad mini, kids are living off their cell phones, the market is shrinking as portable solutions get faster and portable software catches up.
My freaking phone has a 5.5 inch display, 2GB of ram, 80GB of storage and a quad core 1.6GHz processor. What I do for a living requires a pc and I prefer 27" lcd's to my 5.5, but it's not like I break out my laptop on vacation anymore ya know?
I think you're going to see the market move over slowly to keyboarded tablets as people don't need more horsepower, use more cloud storage and want light weight and long battery life.