Since no one else has jumped in, I'll take a stab at it (though I'm still an "inquirer" myself)
As far as I can tell "Grace" has to do with man's participation in God's Uncreated Energies. That way one has a real participation in God's nature and theosis is possible. The western (scholastic) idea is that grace is created, so God and man remain external to each other thus not allowing for true theosis.
I guess you are probably familiar with the Essence-Energy distinction in God. The former refers to His transcedence, which one can never know, and the latter refers to His imminence in the created world, allowing His creatures to experience and know God (as far as creatures can do so). (At least this is what I gathered from reading The Orthodox Way).
Regarding the "divine light", if I'm not mistaken this refers to the uncreated light seen at the Transfiguration on Mt. Tabor and which is a foretaste(?) of theosis which has been experienced by some Orthodox saints in history. It is a manifestation of God's uncreated Energies, as argued by Gregory Palamas, and not a created phenomenon that certain Westerners (the names escape me) claimed it was.
I hope this helps. (And I apologize to the Orthodox here if my explanation is completely off the mark)