The client side object model has been extended to cover much more of the object model that is available server side. Examples are publishing, search, taxonomy, the new translation service and more. A new API for Windows Phone Application is also available. The client.svc is extended with REST capabilities using the OData protocol, support for XML, JSON, etc. is included. One great benefit is that we can use REST for building queries against lists instead of CAML queries.
Here are some links with more information: