I remember once hearing a pastor object to this phrase in the post-sanctus prayer in our Hymnal. It used to be in DS 1 of Lutheran Worship. It is now included in the post-sanctus prayer in both DS 1 and 2 of Lutheran Service Book. He thought that it was a crying shame we couldn't just speak of joy without having to drag repentance in!
"With repentant joy we receive the salvation accomplished for us by the all-availing sacrifice of His body and His blood on the cross."
Is there a gulf between repentance and joy? I don't think so. I think they embrace each other. It is in repentance that we discover joy. Repentance is not primarily grief and sorrow, regret and pain. Repentance is first and foremost a return to joy - the real joy, the joy of God's presence.
"In Your presence is fullness of joy; at Your right hand are pleasures forevermore." Psalm 16
This also reveals to us the true nature of sin: which is turning away from God and what He would give. "But your inquities have made a separation between you and your God." Is 59:2
When we speak of repentant joy, we speak of seeking and finding the only lasting joy in all the world: the joy of the merciful presence of God, His coming to us in grace, His wiping out our sin but not wiping out US. Said most simply: repentant joy is what we have when we turn from chasing joy in all those other places, and find it in the most Holy Eucharist. As our living Lord puts into our mouth His precious Body and pours down our throats His life-giving Blood - there is joy. Forgiveness, for sins are wiped out and gone. Life, for though this Body and Blood are surely the one sacrifice once offered on Calvary's tree, He who gives them to us lives forevermore and so into us go the Body and Blood that death (our death!) can never destroy. Salvation, for living in the merciful presence of God IS salvation.
Repentant joy, then. Let it be so for us each time we approach the Most Holy Table.