Michael Mace has written a very nice article about the problems with Windows 8. It clearly describes the problems I feel the software will face.
The main problem with Windows 8 is that Microsoft wants to leverage Windows to fight the iPad. When you read about the the design goals, you see that Microsoft sees the future as converged devices, with keyboard, touchpad and touchscreen all in one. On the other hand, Apple, which clearly moves OS X into the direction of supporting new features introduced with iOS, keeps the user interface paradigm based on using a keyboard in combination with a touchpad, and improves upon iCloud to ensure the interoperability between them.
The basic problem with the Microsoft vision is the tension over screen sizes. The larger the screen, the larger the battery needed to power it, and the heavier the entire device. The iPad already weights 662g (1.46 lbs), and feels on the heavy side. This means that the larger screens that are needed for efficiently working with classic Windows will be too heavy and even too large to create a reasonable tablet experience. The core benefit of the iPad is that you can use it on the couch, or anywhere else without a table, and that only works as long as the device is not too heavy. But for a desktop, you want the largest screen that you can fit on the table(s) you are using and still be able to carry around.
And an iPad is cheap enough that you can buy it in addition to a laptop or desktop, we are no longer in the nineties, when a reasonable computer was much more expensive, where even when you spent 5000$ on a machine you felt it could be faster for daily work, while nowadays only video editing / computer animation feel slow on a 1000$ computer.
Also mouse based computing has different constraints, it requires different trade offs for the user interface. Metro Apps will always be suboptimal when used with a keyboard, and making classic Windows Apps work with touch requires you to waste a lot of space to make the controls touchable. Take the layout for example: thanks to Fitt’s Law, you will want to put controls on all sides with a mouse, but for touch you will want them together so that you do not have to move your hand around. Or if you have a lot of different tools to present to the user: for the mouse, you will typically show all of them, densely grouped together, and rely on the high precision possible with a mouse so that the user can select what is needed. For touch, you will provide multiple panels with the information shown in a compact way, and when you click on one of these, they will expand to show touch friendly controls to manipulate.
Let us take a very telling example of the difference between touch and mouse: The humble list. On a mouse based device, lists can easily be very tightly spaced, and a tabular grid works very well. On a touch based device on the other hand, you will need 3 to 4 lines heights to pick a line. So you will want to use three / four lines per item, using the extra lines to put info below the header instead of to the right, and maybe use multiple columns of items to avoid wasting too much space. This will mean a different approach to spreadsheets than on the desktop.
So in the end I believe that the approach Apple takes, that you need a different UI for touch than for the mouse, is the right one.
(As an aside, the best way to select text on a touch screen would be to use something like the line loop in Diet Coda, and then use a tap with another finger while you still hold down your primary finger to switch over to switch over to select mode, with the main finger now extending a selection anchored on the position you were at when you did the tap)