My wife couldn't quite pin down what the deal was. She calls the Blessed Virgin "Mary, the mother of our Lord." She said, obviously, that my Lord is God. Quite right.
And this is what St. Elizabeth meant when she created the Virgin Mother: "Blessed are you among women, and blessed is the fruit of your womb! And why is this granted to me that the mother of my Lord should come to me?" (Luke 1:42-43). St. Elizabeth's Lord is, well, the Lord--God.
Lord=God. Kryios=Theos. And in the LXX, kyrios=YHWH.
I have a smart wife. She didn't use the Greek and Hebrew last night, but she knows the score.
So what's the deal, then?
What is wrong in saying that Mary is the Mother of Christ? Absolutely nothing. It's true. What's wrong with saying that Mary is the Mother of our Lord? Nothing. It's great to agree with St. Elizabeth.
But can you, will you, say "Mary is the Mother of God"?
If not, then you don't really believe she is the Mother of our Lord. You mean something different than that. If you can't, won't, don't want to, think it's too confusing, to call her the Mother of God, then when you say that she is the mother of Christ, you mean a different kind of Christ who is somehow not God.
Who do you say He is? Is He God? Was He God in her womb? Or just "the Lord", whatever you mean by that? Was it God on the Cross or just Christ? As if you could separate Him.
Words matter.