Some highlights:

Move to gradle for builds
Redesign SessionFactory building
Introduction of services (see this for more details)
Improved metamodel (not in 4.0.0.Final yet, we planned this, but due to the tasks are more than we expected, and it would take too long to get 4.0 out, so we decided to move this out of 4.0.0.Final but will be upcoming release soon see this for more details, and this is a design document)
Initial osgi-fication by package splitting (public, internal, spi)
Support for multi-tenant databases (see this for more details)
Migration to i18n logging framework (using jboss logging)
JDK 1.6 (JDBC4) as baseline
and more (I can't remember all the things

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