Is it the norm to allow professors from different departments on a PhD committee? Or is this the exception to the norm? I know that it's allowed at Brown - but I wonder if it's unique.
At many (most? all?) universities, a committee is required to have at least one professor from outside the department (other than the chair, if the chair holds a joint position). 

Other requirements are to have at least one professor at rank higher or equal to the rank of the chair (the advisor).