One thing we have to be clear on is that the domain.xml is going to address a small set of what is configurable via the large number of metadata files Brian has listed. The scope of the model is largely going to be based on timeboxing.
I agree with Jason that we need to have an idea of what the admin DSL model is over the long term as the model has to evolve in a compatible way in order for it to be a stable admin API.