Touch screens are not all the same. Even screens that use the same technology (capacitive, resistive, or optical) will show different behavior depending on the controllers used and other details. Windows 8 will have to cope with a wide range of touch implementations, and the Building Windows 8 blog has a new post describing some of the challenges.
The post includes an interesting video demonstrating some of the problems that these variations can cause, with inaccurate taps, misinterpreted gestures, and strange tracking issues where dragged objects run away from your fingers.
Hardware with a Windows 8 logo will be required to meet a certain standard for concurrent fingers, touch precision, and sensitivity. This should prevent many of the issues that can occur with poor touch screen implementations. However, the operating system is still designed to work with existing Windows 7 touch systems in addition to new, Windows 8-spec machines.
To accommodate these machines, Windows 8 has to make some concessions to their more limited capabilities. Machines built for Windows 8 will support a minimum of five touch points. However, none of the core gestures use more than two, to ensure that basic interactivity is always possible. The swipe-from-the-edge gestures are also designed to be sensitive to the restrictions of Windows 7 hardware; they have a 20 pixel buffer at the edge of each screen, so even if the gesture does not start right at the very edge (as will tend to be the case on screens with bezels) it will still count.
This work means that although Windows 8 machines will provide the best touch experience, users of current Windows 7 touch systems such as Samsung's Slate 7 will see the best ever Windows touch experience by upgrading to Windows 8.
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from Ars Technica http://arstechnica.com/index.php
