The most important issue is that however the theist defines "free will", he has the burden of showing that free will by that very definition is supremely valuable: valuable enough to outweigh the great evil that humans (and perhaps other creatures) cause by abusing it, and so valuable that God could not possibly create a better world without it.

This to my mind is the biggest problem with the Free Will defence in all its forms. It seems pretty clear that free will by some definition is worth having; it also seems pretty clear that there are abstruse definitions of free will such that God cannot both create it and ensure it is used only for good. But these definitions don't coincide.

One focal issue is whether God himself has free will, and has it in all the senses that are worth having. Most theist philosophers would say that God does have every valuable form of free will, but also that he is not logically free : there is no possible world in which God performs a morally evil act. But a little reflection shows there are infinitely many possible people who are similarly free but not logically free (so they also have exactly the same valuable free will that God does). And if God creates a world containing such people, and only such people, he necessarily ensure the existence of (valuable) free will but without any moral evil. So why doesn't he do that?

See Quentin Smith for more on this.

You may be aware of Smith's argument, and may be able to point me at an article where Plantinga has acknowledged and refuted it. If so, please do so.