DD are vendor specific because the
EJB container is implemented in different ways. Some (or every )containers provide extra features other than the one specfied in the spec.
Having a common DD would be difficult to achieve. As EJB evolves you will find more vendor specific feature becoming a standard and will fins its place in ejb-jar.xml. for e.g. EJBQL .
If you have a common DD it will be difficult for vendors to compete.
[ October 28, 2003: Message edited by: Pradeep Bhat ]