...from a usability standpoint; and as everyone has pointed out a million times, it's the lack of some type of Start menu. It just doesn't make sense to have to completely leave the desktop whenever you want to launch another application. The silly corners aren't even that big of a deal (although they work much better in a tablet environment than on a desktop).
Win8 was obviously built for a touch screen. Microsoft sees things headed in that direction and decided to make that the primary focus. The desktop gets what feels like a "port".
It's sad too, because other than that Win8 works really well. And it really could have been the perfect desktop. Picture this: You boot into Metro. That's perfect for people who just want to check email or a quick look at the news or weather or what have you. It's good for mouse and awesome for touch screen. When you have serious work (or gaming) to do, you click the "desktop" icon and boom, you have your actual desktop. I mean a real desktop, with its own menu for launching applications. If you want to go back to the Metro it could be a hot corner or even a right-click option. Whatever.
The point is, if you're in the middle of something and need to launch, say, a calculator it's just silly to have to leave the desktop to do it. Sure, you could clutter things up putting the icons everywhere but who wants to do that? I get why MS wanted this for its tablet (how is the Surface working out for you, by the way?) but it seems just plain lazy to not have a true desktop version.
I'd imagine that someone will come up with a mod that essentially adds it back (actually, there already is one, although all it does is put metro in a window on the lower right of the screen where the Start menu would be--not ideal, but it shows people are thinking about it) or maybe even MS will realize that it's best to give people what they want.
But then, everyone knows you skip every other version of Windows anyway.