Watching the debate between Douthat and the liberal theologians has been rather instructive. Conservatives are interested in demonstrating from Church tradition that their position is correct and obligatory. Liberals just don't seem to be interested in engaging this argument. For them, the general facts that the Church has changed in the past and that theology and history are complex are apparently adequate for putting aside just about any argument from tradition. They want a certain result, and if there are specific discontinuities with the tradition, they are happy to let the professional theologians paper over them.
Intelligent and educated conservative Catholics like Douthat are quite aware that the Church has changed, that doctrine has developed, and that history doesn't lend itself to an easy argument for a broad understanding of the infallibility of the Church. For liberals, this historical consciousness has resulted in a kind of demythologizing of the tradition of the Church (or, at least, of all but its very core) and of any arguments made from it. Arguments from tradition were used in the past to claim that the Church never innovates, but simply reiterates what has been handed down, but this can be demonstrated to be false, so these arguments can no longer be made in earnest. At best, they are a form of discourse for governing how the Church changes to meet changing times. But they do not determine the result. The result is determined by the needs of day. For historically aware conservatives (i.e., not "fundamentalists"), though, this demythologization has not occurred (or has not proceeded to the same extent). They still see fundamental continuity in the development of doctrine as a whole and not just in the very core of Church teaching. Thus, they still make arguments from tradition in earnest.
Unlike the liberal, what they seek primarily in such arguments is not the "best" solution as determined by the needs of the day, but the best expression of faithfulness to the tradition. If there is innovation, it needs to be continuous with the tradition and come out of it--a clarification or synthesis that takes better account of the data of tradition. (This is how non-traditionalist conservatives view Vatican II.)
Liberals seem to be content with showing that a proposed innovation meets the felt needs of the day and does not contradict a clearly infallible teaching of the Church. It is not that conservatives view all magisterial statements as infallible, but they put the burden of proof on the innovators and that they regard the tradition as always authoritative, even if not infallible. This means that, if past magisterial statements are to be revised, they should be revised from within the tradition and from higher in the "hierarchy of truths," not just based on the perceived needs of the day.
