Just a brief note of frustration. Why is it that whenever you mention Balthasar to a Barthian, they inevitably (almost nervously) remark that Balthasar was much indebted to Barth, but not vice versa? It is true that Balthasar was more influenced by Barth than the other way around. But it is not clear to me that Barth was a very strong influence (meaning: source) for Balthasar's theology. If Balthasar was more open to Barth's theology than vice versa, it may say as much about Barth's modus operandi as it does about Balthasar's indebtedness to Barth.
Although it is true that Balthasar once said of his major triptych, "I wrote it all for Barth," thus perhaps suggesting an indebtedness to Barth, one should not ignore the second half of the statement, "to convert him." Balthasar, I suggest, was not interested in Barth because he was the genius of his age (as most Barthians would contend), but because Balthasar saw in Barth the finest, most Catholic Protestant theologian Christianity had known. I suspect Balthasar's interest was ecumenical and conciliatory, and that he drew on Barth as a kindred theologian from the Protestant side in that capacity. Now doubt, Balthasar admired Barth for heroically uprooting the tracks to Protestant theology just as he admired the nouvelle theologie for waking from the Thomistic slumber of Catholic theology (on which, see Fergus Kerr's excellent book).
I recently discovered a fine review of Stephen Wigley's book, Karl Barth and Hans Urs von Balthasar, by my friend John Drury. Wigley concludes that Barth is the key influence on the eventual structure of Balthasar's triptych. John rightly takes him to task here, mentioning the theories of McIntosh and Mongrain who find more of Maximus the Confessor, Ignatius and Irenaeus in Balthasar's thought. Each of these has more claim to a central role in Balthasar's theology than Barth. One influence John also failed to mention, however, is the key influence that Balthasar himself cites: Adrienne von Speyr. Now, Mongrain has taken Balthasar's claim to task in his attempt to demonstrate Ignatian and Irenaean themes throughout Balthasar's works. Even so, if anyone had a claim as a key influence, surely Adrienne would be it. Even more so, we might want to point to those who introduced Balthasar to Irenaeus and Maximus--his teachers Henri de Lubac and Jean Danielou (just to name two). These figures of the so-called nouvelle theologie are, no doubt, every bit as strong of influences on Balthasar's thought as Barth. So, we should keep in mind that simply because someone writes something for someone does not necessarily mean that have written it under the influence of that someone. It has often been noted that Balthasar wrote his triptych in reverse order as Kant presented his own. I do not think we must therefore conclude that Kant is the key influence in Balthasar's theology.
I know it is hard to believe that Karl Barth might not have influenced profoundly every theologian of note, but it is just true. Barth was a genius in many ways, and certainly (at least to my mind) the prince of twentieth-century Protestant theologians. But let us resist finding his genius everywhere and make room for others of similar genius. Balthasar was one of these. He was an incredibly talented, creative, and original thinker. In reality, his influences lie with the whole of the German intellectual tradition (especially Idealism), with the Church Fathers, with French literary figures, and the list goes on. Balthasar devoured philosophers, theologians, literary authors, and composers with awe-inspiring capacity and range. While Barth, as the greatest Protestant theologian of his age, was inevitably one of those influences, he was not the only, nor even the main influence, impetus, or source for Balthasar's theological writings. The reductio ad Barth is a severly diminished understanding of Balthasar's mind.
And so, I offer a gentle plea to Barthians to read Balthasar on his own terms, and not through Barth-shaped glasses. I think the exercise is well worth the exertion.