Because 2.1 is a maintenance release of the JSF specification, there are few big ticket new features.
This feature enables files written in the JSP Document Syntax (.jspx files) to be treated as if they were Facelets files. This approach, in concert with migrating your JSP custom tag libraries to Facelet tag libraries, will ease transition from JSF 1.2 to the latest version of the specification.
This API allows the in-memory cache of Facelet instances to be served from a cache that can be overridden with a custom implementation.
Both Mojarra 2.0 and MyFaces 2.0 implemented this feature but it is not specified until version 2.1. This feature leverages some new API in Servlet 3.0 to declare default mappings for the FacesServlet. As a result, the web.xml feature is no longer necessary if you are satisfied with the default mappings declared in the specification.