Protestants and Catholics actually agree on the key thing here: namely, that the authority of Scripture is the authority of God, that the authoritative teacher of the meaning of Scripture is the Holy Spirit, and that without Him there is nothing but darkness. The difference arises in that, from the Protestant point of view, Catholics too easily conflate the interpretive act of the Holy Spirit with human interpretive acts, and, from the Catholic point of view, Protestants have an incomplete view of the work of the Spirit in the Church's interpreting of Scripture. As long as the dispute never seriously engages with this point, it is self-perpetuating, because it will never have any effect except the raising of even more light-obscuring dust.
I appreciate Brandon's concern to emphasize the similarity more than the difference. But I cannot really agree with his description of the difference. As is so often the case among philosophers, the disagreement is not so much over the answer to the question as over the content and import of the question itself.
I just don't think it's the case that, "from the Protestant point of view, Catholics too easily conflate the interpretive act of the Holy Spirit" in the reading of Scripture with "human interpretive acts." To my ears, a far more typically Protestant criticism is that Catholics too easily conflate the Church's interpretive acts with the interpretive acts of the Holy Spirit, when the Church's are presented as authoritative. The criticism is not so much that Catholics conflate God's interpretations with man's, but rather with the Church's; most Protestants would not mind seeing more individual interpretive acts by Catholics, unconstrained by the Church but somehow discernible as Spirit-led. As for "the Catholic point of view," I should say as a Catholic that the problem with a Protestant approach to interpreting Scripture—insofar as such a thing is both identifiable as such and distinct from a Catholic approach—is not so much its incompleteness as the baleful and inevitable consequence of that incompleteness.
When we interpret the Bible as individuals, our interpretations will always be "incomplete" and non-authoritative, even if our interpretation is Spirit-led, as it sometimes is. Even Catholics can and do read the Bible that way, more than they once did but still, perhaps, not as much as they should. We thus gain spiritual insight as readers of God's word. But lectio divina of that generic sort is authoritative for others only insofar as it is in harmony with interpretation that is authoritative for other, identifiable reasons. Even when we read the Bible as members of a church with some sort of tradition, a churchly interpretation cannot be considered authoritative for and binding on believers if the church in question no more claims infallible teaching authority than any individual within it. That is why I believe Jonathan is right when he says: "[I]f there's no authoritative interpreter, then there is no authoritative character of what is interpreted." For Protestants who acknowledge an infallible rule of faith, the Bible is it; and the only "authoritative interpreter" of the Bible is the Holy Spirit inscribing his wisdom in our hearts as we read and hear the Word with docility and prayer. But appeal to an authoritative interpreter of that kind is simply idle as a way of adjudicating among conflicting interpretations that are doctrinally significant. Each side can and will claim the Holy Spirit for itself; absent appeal to an identifiable tradition and a visible teaching authority, nothing other than subjective intensity and academic opinions can be offered as evidence for one side or the other. As a way round that, some Protestants, claiming "by their fruits ye shall know them," are prone to appeal to what outstanding Christians would say; but given that the testimony of Catholic (and Orthodox) saints and doctors usually fails to weigh with such people, I am unimpressed with their appeals to this putative consensus.
Now Brandon's account of "plain sense" gains such force as it has by suggesting that a distinctively Protestant way of reading Scripture, which the Catholic would indeed consider incomplete, can nonetheless yield insights to which the Holy Spirit is leading the ordinary person. He says:
...the plain sense of Scripture is a fairly minimal doctrine -- it just says that Scripture has a literal text open to ordinary people; and as it is usually used, it suggests that someone reading the literal text can, without profound education, be led through that text by the Holy Spirit to Christ, and to deeper intimacy with him.
But as a way of making the idea of "the plain sense of Scripture" both clear and plausible, that seems to me at best unhelpful. Having earlier denied that "the literal sense open to ordinary people" is, necessarily, that which the human author(s) intended, Brandon must say that the plain sense is what the Holy Spirit enables the ordinary person reading Scripture, precisely as such, to discern. But such a "literal" sense seems not at all as distinct from a "canonical" sense that can often go well beyond what the human author(s) intended. It is of course quite possible for ordinary believers to discern such a thing by reading Scripture; but it is equally possible for them not to. And that, in a way, is the key point. Discerning such a thing by reading Scripture on one's own as a fallible individual, or as part of an ecclesial community that does not claim infallible teaching authority, can enable one to discern what is of faith; it can even be Spirit-led; but it can only elicit assent by faith if the sense of Scripture one so discerns is in fact recognizable as authoritative for other reasons. That is why Vatican II teaches, in Dei Verbum §10, that the Magisterium is the only "authentic" interpreter of Scripture. The teaching does not mean that only the Magisterium can come up with interpretations that are true or helpful; as a matter of fact, the Magisterium will sometimes adopt interpretations arrived at without its intervention. The teaching in question means that only the Magisterium can authenticate, and thus render authoritative and binding, what Brandon calls "the plain sense" as the sense intended by the Spirit.
Frankly, I still don't understand what work the idea of "the plain sense of Scripture" is supposed to do, other than cutting out that proverbial middleman in Rome. But the result, for reasons I've often adduced in the past, is unreliable. Without reliance on both Tradition and the Magisterium in one's reading of Scripture, what is so often thought to be the plain sense is not so plain as all that.