We can bring the idea of "metaphysics" down to earth by relating it to the idea of "semantics." If metaphysics is the study of what exists (in our time this is essentially the confrontation with materialism), semantics is the study of the meaning of words. If my friend is talking about "angels" I can think about whether such putative entities exist but, more subtly, I can ask what he means, or aims to communicate, by this word. Thus even if we tacitly accept (as many of our contemporaries do) a physicalist axiom (metaphysically speaking), that doesn't mean that there is no longer anything to discuss about the mind/body problem. In fact the semantic analysis of intentional and phenomenal terms (the psychological vocabulary) remains an open and even a pressing issue, even for the thoroughly modern physicalist.
For Descartes the mind/body problem was essentially an interaction problem. He did not question (that is, he had his reasons for accepting) the existence of both physical substance and mental "substance." The metaphysical problem as he saw it was about causal relations between them. Thus there was one sort of entity, body, and another, mind. But if we don't accept Descartes' underlying ontology the problem is altogether different. Specifically we needn't see "mind" as referring to one thing or having one meaning (this was Ryle's enduring point expressed in the very title The Concept of Mind). Once we see this we can take a crucial step: we can distinguish the intentional psychological vocabulary ("belief," "desire," etc) from the phenomenal psychological vocabulary ("pain," "sensation," etc). We can see that there are (at least) two metaphysical (semantic) problems here, not one.
To apply this, I think that the eclipse of reductive materialism in favor of functionalism on the grounds that intentional states superevene on (are multiply realizable in) physical states is justifiable (there is indeed a problem for reductive materialism here), but that doesn't preclude identity theory as applied to phenomenal states. And that insight opens up a whole new discussion in philosophy of mind.