Jack Good, while discussing in The Dishonest Church the differences between what he calls progressive Christianity and what he calls popular Christianity, mentions the dangers that both forms of the faith can pose:
Popular Christianity at its extreme becomes judgmental and rigid. Witch burnings, inquisitions, and religious wars are too frequently the bloody, tragic results. Progressive Christianity carries another set of dangers: Unless care is taken to see that it is deeply and strongly rooted in an ongoing tradition, it becomes a watery system with few defining commitments, a wide but thin blanket placed over ideas that are often distressingly shallow.Personally, I think that witch burnings and inquisitions are a vastly more serious danger to the world than a watery religion, but he does raise a valid point. Progressive Christianity could, if not rooted in Christian tradition, simply evolve into another iteration of Unitarian Univeralism.
That isn't to say that Unitarian Universalism is a bad thing. It serves the needs of a body of adherents, and as such offers a legitimate role in American religion. I myself found the UU denomination to be an entry point back into religion after years of atheism. But this is not the direction that I would like to see progressive Christianity move in. It is interesting to consider what happened in the history of the UU faith. Having originated as a liberal strain of Christianity (well, actually two liberal strains--Unitarianism and Universalism), it nevertheless over time became so attached to the idea of free thinking that it divorced itself from the Christian tradition altogether--to the point where only a small minority of its adherents now consider themselves Christian at all. And, despite an ostensible commitment to tolerance and diversity, many UUs are actively hostile to Christianity in any form.
The key is, as Good points out, staying rooted in the Christian tradition. There is, I believe, a body of faithful believers who want to remain within the Christian tradition but who also want to practice a thinking, non-dogmatic version of the faith that accepts modern science and serious biblical scholarship. This is what progressive Christianity should be about, and it is these people who have left many mainline denominations behind because those denominations did not satisfy this need.