What does it mean to say that God is perfect? St. Thomas says that God is perfect because there is no potentiality in him: we may not say, for example, that God's justice is lacking, nor that he does not love us fully, nor that his goodness has limits.
God is the first principle, not material, but in the order of efficient cause, which must be most perfect. For just as matter, as such, is merely potential, an agent, as such, is in the state of actuality. Hence, the first active principle must needs be most actual, and therefore most perfect; for a thing is perfect in proportion to its state of actuality, because we call that perfect which lacks nothing of the mode of its perfection (ST I Q4 A1).Because there is no potentiality in God, he is perfect. This is a bit hard for me to wrap my brain around, because it is so very unlike us creatures.
But just as every excellence and perfection is found in a thing according as that thing is, so every defect is found in it according as in some way it is not. Now, just as God has being wholly, so non-being is wholly absent from Him. For as a thing has being, in that way it is removed from non-being. Hence, all defect is absent from God. He is, therefore, universally perfect. ...Since there is no sense in which God lacks being, there is no sense in which he could be said to be lacking something. Consequently he must be perfect. And since God is pure act, without potency to be actuated by any outside cause, he is also perfect in this sense as well.
Again, each thing is perfect according as it is in act, and imperfect according as it is in potency and lacking act. Hence, that which is in no way in potency, but is pure act, must be most perfect. Such, however, is God. God is, therefore, most perfect. (SCG I, 28, 3 & 6).
It seems to me that one consequence of this has to do with our understanding of the way that the Bible speaks to us of God's interaction with men. We see that it says in places that God became angry with certain people because of their sins, or that he was pleased with them because of some good thing that they had done, but these expressions must of necessity be exactly that: expressions. Because there is no sense in which we can rationally say that we can cause a change in God by anything that we do. We can say of ourselves that we had a really lousy day because the kids were a constant pain, or because our employees were lazy bums all afternoon, but we can't give God a lousy day. So it seems to me that such expressions must be understood as accommodations to our limited comprehension: God is said to "react" in these ways - becoming angry, for example - not because it really happens that way but because it's the best we can do even to try and understand what is happening. God is perfect, and part of that means that there is no potency to wrath in him that could be actuated by us. He is perfect, and consequently there is no defect in his perfect bliss and joy that can be corrected by any good thing that we might do. So the best we can hope for is some figure by which we might understand God - hence, his wrath with sin, his pleasure with holiness.