And JBoss AS 5.0.0 moved from a JMX Kernel base to a microcontainer base, thus most services are POJO-based. Of course, one of the primary services is a JMX servies which then hosts those services that have not yet been converted to the microcontainer.
In general, JBoss AS is a certified
Java EE 5 application server. As such, it is required to provide a large number of services (see
http://java.sun.com/javaee/technologies/ for a list) and uses various other projects to provide many of those services.
Check out chapter 1 of the book, it is a free download. That has some information on why to use JBoss AS. But we do not go into a detailed comparison with other app server.