Useful post on MSDN titled Windows 8.1 - New Features in XAML.

•Theming - Dark and Light themes
◦In Win 8.0, one can change the Dark/White theme once.
◦In XAML , set RequestedTheme="Light". That element and all of its child elements will follow that theme.
◦However, HighContrast setting in the Control Panel will override this.
◦Created ThemeResource. Replace StaticResource in Wind 8 code with ThemeResource term.

•Hub Control - Bing Travel App like Experience, New Visual Studio project template.
◦List view and Grid view for a vertical and horizontal layout
◦Hero Image and followed by Hub Sections (Content Control)
◦Deferred loading of Hub Sections. Don’t have to wait for all the hub sections to load.

•Styles - StandardStyles.xaml. We broke every framework Style into to its own and added them into the Framework. Don’t need any base text definition in the app anymore.
◦VS has intellisense on StaticResource now. VS will also enumerate the Styles available in your app and also in the framework.

•CommandBar- New Command. It has Primary (right Side) and Secondary Commands (left Side).
Full read.