No doubt JAX-RS 2.0 (JSR-339) has been, is and will be a success - a lot has been written about the top features JAX-RS 2.0 offers. It is still very much a relevant story for many developers who have their REST services being migrated to JAX-RS 2.0, it is not always easy for a given production to switch to a new specification's API fast.
But JAX-RS 2.0 is not the end of JAX-RS as such. So the fact JSR-370 (JAX-RS 2.1) is now active is a very good news for all of us working with or interested in JAX-RS.
Have a look at the "Request" section and check the list of the improvements and new features that the specification will cover. Good stuff. Note the effort will be made to have JAX-RS applications much better prepared for supporting Web-based UI frontends. Another thing to note is the fact it will be Java 8 based so expect Java 8 features making themselves visible in JAX-RS 2.1 API, Marek and Santiago will come up with some very cool API ideas.
All is great in the JAX-RS space. Explore it and enjoy !