I'm reading Hans Urs Von Balthasar's "Love Alone is Credible" and Karl Rahner's treatment of the Trinity, as well as considering the theology of Karl Barth, as well as that of Confessional Reformed Protestantism. Add to that Jaroslav Pelikan's last volume of Church History (1700-Vatican II) and you'll see where I'm drawing this.
In "classic" / Reformational differences, one important divergence seems to be this:
Reformed thinkers argued for a view I think best reflected by Cornelius Van Til's presuppositionalism. ie. You must accept that God exists and that the Bible (66 books) are divinely inspired. Until you do, I will undermine your worldview over and over and over, ad infinitum, until we get to my worldview. So eventually, everyone will accept sola scriptura, and when they open the bible they'll see double predestination, accept Calvin, and be saved, and we'll all live happily ever after.
This debatably stems from William of Ockham's rejection of Scholastic Realism in favor of Nominalism and the denial of universals. Thus Revelation and Faith were placed in opposition to Reason and Metaphysics. (though some Reformed thinkers have actually used Alot of Aquinas and to their credit, have come up with some good philosophy)
Roman thinkers have oppositely argued that the divine image implies a use of reason and that by using 'faith seeking understanding' and philosophical theology, metaphysics, etc, everyone in the world will eventually give their life to Aristotle, and come up with the Summa if they just try hard enough. And we'll all live happily ever after. (Although the Augustinians and some others like Bernard of Clairvaux have always rejected this kind of thing, arguing much more like Pascal that 'the heart has it's reasons' which the mind can't know).
The sticky situation remained unresolvable, until recently.
Karl Barth came up with a beautiful exposition of nonfoundationalist Protestantism (partially Lutheran, partially Reformed), that caught the eye of Hans Urs Von Balthasar who came up with a beautiful exposition of nonfoundationalist Protestantism. Both of them kind of gave in to Van Til's method - a bit, not completely, you must note well - and argued for the supremacy of Scripture and Christ to EVERYTHING. Yes, even to some of our beloved traditions. Henri de Lubac apparently did something similar. Dei Verbum - the dogmatic constitution on Scripture in Vatican II gave over the Catholic hypothesis of the 2 source theory (Scripture + Tradition) in favour of the East Orthodox (Scripture as Tradition) theory, thus moving us another inch closer to Protestantism.
Now, in the face of Emergent church craziness, and Eastern Orthodox and Roman Catholic threats, the Reformed folks have kicked into high gear by reviving the "Tradition" part of their Tradition. Jonathon Edwards, John Calvin, and Martin Luther's books have been selling like hotcakes (except unlike these - which I've never seen sold, they're actually selling). Reformed Theologians are appealing to their creeds rather than "the Bible only" and using things like Chalcedon to smite Mormons, Unitarians, and American Millenarian Evangelicals and finally noticing that they're using Tradition.
Thus it seems Reformed Theology has stepped closer to Rome.
I'm highly uneducated, and have only been involved in theology for 2 years or so, but it seems to me, that all this nonfoundational epistemology could mean new ecumenical possibilities for these 2 Traditions.
In a slightly different context, I discussed this with a Reformed Philosopher at my university the other day, and just by changing the philosophical groundwork, we were able to come to an agreement on huge issues. In that friendly environment, he admitted that Reformed theology functions greatly out of Tradition and that Sola Scriptura in it's sharpest understanding holds them back from alot, and I admitted that Catholic theology functions on the principle of claiming infallible statements, but then re-interpretting/defining them to the point where they are unrecognizable anymore.
Thus saw I the great influence of philosophy in theology. It undergirds everything.
So perhaps, if we could ever reach this kind of honesty, away from the glorification of either the Reformation or the Great Catholic Tradition, and simply 'gather round the cross' as Spurgeon used to say, we could come a few steps closer to reunion, even if nothing greater than that gets resolved.
But that will never happen, inevitably polemics will continue, new papal decrees on formerly heretical titles for the Virgin will be promulgated (*cough assumption*), and new books by R.C. Sproul and Michael Horton will be published against N.T. Wright and anything that moves against Traditional Sola Fide. And our children's children will be forced to continue these debates till Christ returns and tells us the East Orthodox were right and that all of us Western Heretics will be making license plates next to Origen, Erigena, and Marx for eternity, while our enthusiastic Greek neighbours begin to cross themselves backwards... (but that's just a guess).