This week's book giveaway is in the OCMJEA forum. We're giving away four copies of OCM Java EE 6 Enterprise Architect Exam Guide and have Paul Allen & Joseph Bambara on-line! See this thread for details.
I feel that is the area where they added the most classes and methods. They have provided an alternate set of classes to address IO and have done a pretty good job. It is definitely one of the most significant changes. Of course, the significance of a change is dependent on your needs.