Good question :-)
The JBoss Admin Console is a web-based application aimed at making the job of administering JBoss instances easier. It will provide the ability to use a browser for performing common tasks such as creating JMS destinations, changing DataSource settings and updating Security domains. Think of it as being the JBoss equivalent of the admin web applications that come with WebLogic and WebSphere.
MC4J is a very different tool, it is a Java based desktop application which enables access to MBean attributes, operations and notifications. It is aimed at working with any JMX enabled system, e.g. J2SE 5.0 JVM, and so is not geared specifically for administering JBoss. For JBoss administration the Admin Console has an advantage since it makes use of specific knowledge of how JBoss works to make tasks simpler. For instance try changing the username or password for a DataSource via JMX. Using the DataSource MBeans this is tricky since the information is stored in a org.w3c.dom.Element, also it's not possible to make the change persistent across server restarts. The JBoss Admin Console provides a page where you can review the current DataSource settings, update the username or other attributes, and hit the Save button to have your changes made. Such changes will also persist across server restarts.
Currently the JBoss Admin Console provides the management of DataSources i.e. create, read and for DataSource created through the Admin Console update and delete. Please checkout out JBoss HEAD...
cvs -d:pserver:email@example.com/cvsroot/jboss co jboss-head
and take a look at the documents in jboss-head\admin-console\docs for a more precise description of the functionality.
The code itself is in alpha state, meaning that all the functionality is there (as described in jboss-head\admin-console\docs\requirements\use_cases.pdf) and it has been unit tested, but it has not yet been QA tested. Please try it and send feedback, look at jboss-head\admin-console\README.txt for building and deployment details. We've just started the next phase of development so keep an eye on this forum for new features.