As you move up the JBoss versions, you will see it upgrades the capability to support the changes in the EJB specs so for the recent JBoss 3.2.x - EJB 2.0/CMP 2.0 and JBoss 4.0 - EJB 2.1. It has additional infrastructure support for JMS, JAAS framework and a microkernel architecture so you can install MBean plugins - so any custom service you need that doesn't exist you can build.
Servlet containers are borrowed from existing external projects such as Tomcat and Jetty, rather than re-0inventing the wheel.
The only specific Postgresql configuration required is the dataservice and there are plenty of examples in the Datasource configuration in these forums. The configuration for the latest versions of JBoss are fairly simple. More complex issues like entity beans are best dealt with in the forums if you can't work out how they operate. Apart from that, the EJB layer is insulated from the actual datasource so a datasource is a datasource is a datasource.
The jmx-console gives you a view into the sub-systems that are operational. I think this has been improved in JBoss 4.0. At this point, no GUI console editor to change configuration is available. But the XML configuration scripts are simple enough to manage IMHO. Deployment is drag-and-drop so to speak - place the deployment package in the deploy directory and JBoss will take care of automatic deployment. Having had experience with SilverStream and WebSphere, I'd say that the JBoss deployment process is faster, simpler and more transparent.
That's a starter.