According to Danny Coward's Sun Weblog there are several important enhancements:
Easy to use APIs for developing web service clients.
Updated JDBC 4.0 APIs, and an all-Java JDBC database included in the JDK.
New Desktop APIs
Swingworker, JTable sorting and filtering, GroupLayout and more.
Monitoring and Management
All the m&m tools now attach on demand. And there's a new one: jhat.
Programmatically control the javac compiler.
Define your own annotations and plug in the code to process them.
Swing's better looking and better accelerated. Revamped runtime and application installation.
Integration with services like PKI, Java GSS, Kerberos, and LDAP.
Performance and Quality
Double digit improvements in performance, client and server. 100,000+ tests for compatibility alone.