SharePoint 2013 offers a whole new set of design opportunities to designers and developers.
The basis for this wohle system is the well known masterpage. The masterpage is now beeing extended by the possibility to import own developed design frameworks (called design files), usually simple sets of html-files with images and CSS. These design files are converted to masterpages which then can be connected to the Sharepoint Snippets, such as Search box, security trim, site logo a.s.o. Also custom ASP.NET markup can be placed.
One of the main design challenges in the upcoming years would be optimization of web interfaces to different devices. Therefore Microsoft implemented something called device channels. With device channels different devices can be adressed with different masterpages. Also device channel pannels can be implemented via a snippet in the masterpage. Understanding modern web technologies and the possibilities provided from Microsoft the it will be necessary to implement new templates on the basis of HTML5/CSS3. Also responsive webdesign should be covered within a masterpage.
Summing up SharePoint 2013 comes with a bunch of new functionalities within the design issues. New functionalities should not override ideas coming from modern webdesign principles: on the other hand it should be highlighted that the focus not only should lie on responsive design – an effective mixture should be favoured.