Barth attempts to explain in his Church Dogmatics, that the problem of justification consists in how man can be at the same time sinner and just.  Therefore, Barth surmises, the doctrine is unique and important.  Here are some excerpts: 

How can he be simul peccator et iustus?  And how can God for His part (the omniscient and righteous Judge of good and evil) give right to man when man is obviously in the wrong before Him, and God Himself has put him the wrong? … To what extent is this justification not a mere overlooking or hiding of the pride and fall of man, a nominalistic “as if”—which is quite incompatible with the truthfulness of God and cannot be of any real help to man—but God’s serious opposition and mighty resistance to the pride of man and therefore the real redemption of fallen man?  How in this justification can God be effectively true to Himself and therefore to man—to man and therefore primarily to Himself?  How can He judge man in truth and even in that judgment be gracious to Him?  How can He be truly gracious to him even in the fact that He judges him?  This is the problem of the doctrine of justification which we now have to develop.  (CD, § 61: 517)

…

But whether we are dealing with a divinely true actuality depends upon whether in this alteration of the human situation in the atonement—as the work of grace and mercy of God—we are dealing with that which is just and right.  It depends upon whether—however strange it may seem to us—there is a genuine justification: that is, whether the right of God which gives right to man and the right of man which is given by God to man is a true and indisputable right.  If we do not have an indisputable divine right, and (for all its difference) an indisputable human right, how can the conversion of man to God be true, and how then can it be actual? … [The task of the doctrine of justification] is the task of finding a reliable answer to the question: What is God for sinful man?  And what is sinful man before the God who is for him?  The basis of the community and the certainty of faith stands or falls with the answer to this question. (CD, § 61: 518)

…

There is no doubt that the unusual difficulty of the doctrine of justification is an indication of its special function.  In it we have to do with the turning, the movement, the transition of the existence of man without God and dead into the existence of man living for God and therefore before him and with Him and for Him. … There is no part of dogmatics, no locus, where we can treat it lightly.  At every point we are dealing with the one high Gospel.  What we can and must say is that in the doctrine of justification we are dealing with the most pronounced and puzzling form of this transition because we are dealing specifically with the question of its final possibility. … But in the doctrine of justification we have to do with the original centre of this crisis, and to that extent with its sharpest form, with what we can describe provisionally as the crisis which underlies the whole.  If we find it running through the whole with all kinds of repetitions and variations, at this point where we grapple with the peculiar difficulty of it, it has to be seen and handled as the main theme—the question: How am I to lay hold of a gracious God?  And it is from here, and along the line which runs from here, that in different ways it works out everywhere. (CD, § 61: 520-21)