What are the major things preventing this?
No effort has been made to not depend on org.jboss.* classes and the MBean server stuff. Other application servers do not have the concept of service dependencies. Its not a project priority at the moment. Once the "microcontainer" project is mature we will probably port to that framework, but in the interest in releasing 1.0 by Q2 and having something thta is stable and works all along from now till then (JBMS is stable/works/etc as is), we make compromises and avoid stuff that is not stable (having your service interfaces RIPPED out from under you every release would be the very definition of NOT STABLE).
It is theoretically possible (but may be silly) to bootstrap the JBoss Microkernel from JBAS on another appserver (its just java) provided you can keep their JMX implementation out of the way. However, if you go that far you might as well run JBAS. But the last part is mere opinion.