I like the employer/employee illustration. I submit that it may be a bit simplistic for the description of relationships.

Power seems to be as a word too vague. It describes the possession, by one party, of a thing of value desired by the other party. Me having, and you wanting, is leverage. I now have the ability to compel you, to a degree. The value that YOU place on the thing I possess determines the extent of that leverage - how far I can push.

This simple dynamic describes employment, because -generally- in a work environment the employee is replaceable.

In a relationship both parties have leverage. Her desires place value on my strength, stability, attention... My desires place value on HER stability, submission, ability to compartmentalize...

Codependency is another way of saying mexican standoff. If I did not value my sub's emotional welfare, I would have all the leverage. I would also be a douchebag. ^.^

Even situations where the sub claims to have no self regard seem to me to be false, because that in itself is a trait that any dom could value, and thus the sub has gained leverage... Power...

Occam's razor. It is called a power exchange because it is, and in a very literal sense.

Pardon me; I often use typing to think.
-matin