One of the new features in 4.1 will be the addition of an actual API for loading entities by natural id. Yes previous versions had the ability to do natural id loading leveraging criteria queries, but that approach was very limiting. The new API will allow caching at both the Session and SessionFactory level in addition to providing a consistent API. The new approach has been made available for identifier based loading as well, again for consistency.
The GitHub repo formerly named hibernate-core will be getting renamed after the 4.0.1 release today. Its new name will be hibernate-orm. The whys have been discussed over the past few weeks in the developer meetings, so I will not go into all that; see the meeting minutes if you are interested. I did, however, want to discuss the ramifications of this rename.
4.0 changes the way event listeners get registered to leverage some added capabilities, namely services and integrators. I won't get into services too much as they are covered elsewhere. But lets start with the notion of an Integrator.