JBoss Release Notes
The Release Notes for JBoss are generally published as change notes on Sourceforge. They are located in the documentation section on Sourceforge, and now on JBoss Jira under the Change Log of each project.
Some of the addtional features may have corresponding Wiki-pages. They can be linked from here.
batch-commit-strategy (see cmp2.x jdbc2 pm)
General 3.2.x to 4.0.x Migration Notes
See ConfigDataSources for the query timeout feature for DataSources
See ConfigDataSources for the background-validation feature for DataSources
New transaction manager: JBossTransactions
Optional Apache Portable Runtime Support added, which gives you native (non-Java) HTTP connectors
Also check the JBoss 4.2 FAQ
Updating this document
If you find a feature that is mentioned in the release notes on SF/Jira and a corresponding wiki page exists, please take the time to add it here. Add that feature to the first release that introduced that feature.