This is by no means a complete list but just some observation on files that caught my eye. Some are just regular header file updates for Windows 7 and does not necessary require Windows 8. Some other APIs moved to header files with more readable names.
Improvements are focused, of course, in natural input, efficiency, gaming experience and IE. You can hear the silence scream of “Tablet!Tablet!Tablet!” here. Note Windows 8 is still a developer preview and there is no guarantee any new feature will survive the final cut or there won’t be new API for existing features add in Beta/RC/Gold.
There is no improvement in the GDI-based shell common controls. No surprise here. The new UI would be based on WinRT and will use GPU to render instead of CPU. There are COM API for printing, which is traditionally performed via GDI.
A lot of Direct2D and Direct3D improvements. XAudio2 added and battery/pad support added to XInput. Improvement in 3D rendering, device removal during capturing and stream seeking in Windows Media Foundation. API for H264 codec.
Header guard for IE5.01 or lower are removed. Building for Windows 2000 is now practically impossible. Some header guard switched to NTDDI_VERSION. There is A new camera UI control based on COM. Somehow the version guard of Shell_NotifyIconGetRect is lost, this might be a header file bug.
Lots of IE 10 APIs and HTML 5 support. ActiveScript optimized for multiple instance and webworker. Also there is a new IActiveScriptContext interface to get url, line and offset of the script. New timeout configuration in WinInet. ActiveX filtering, and options to enable/disable audio/video plugins added for IE security zones.
Accessibility for windowless controls and lots of UI automation improvements, including grid and spreadsheet types. Candidate list integration and async document support with Text Service Framework. ARM support to .Net APIs. A lot of pointer device and touch messages, notification and drawing.