19th Century Protestantism developed in a few parallel streams as it responded to the historical situation. We all know what was happening: the Civil War, industrialization, Darwin, Marxism, Freudianism, feminism, etc. Protestantism became different streams: Fundamentalism, Pentecostalism, Evangelicalism, the Mainline. Unitarianism and Universalism were two of those streams of Protestantism, quite distinct from the othes.
What gives the Uni's their unique character was their willing confrontation with secularism and humanism. Instead of trying to repel them, or protect the historic faith from them, they embraced humanism. The mainline Protestants, on the other hand, did not embrace humanism, but onlyaccommodated skepticism. That is why they have this two-tone quality -- on the surface, they are still orthodox, but in practice and in the pews and in the pastor's study, they are much more liberal.
The Unitarians of the early twentieth century went another way. They quite gleefully tossed out the last vestiges of Christian orthodoxy they still held. In most of the country, Unitarianism became 'the atheist church', long before these 'Sunday Assemblies' got started in London. Unitarianism in its non-New England religious humanist form retained the organizational structure of churches, Sunday morning services and an ethical/moral system distilled from the Christian tradition. And Christmas in most places, but not Easter.
If it were not for the persistence of New England Unitarianism (Christian-ish, theistic, culturally mainline Protestant) and the ministers steeped in that tradition, Unitarianism would have become whole-heartedly humanist.
I think most of what goes on in Unitarian Universalism are efforts to harmonize our Protestantism and our humanism: theologically, evangelically, ecclesiologically and liturgically.
Protestantism and Humanism is an uneasy marriage, an unstable combination.
Out of all its offsprings, I think that the line represented by people like James Luther Adams is the most creative and unique. He argues that the salvation history of humanity moves into the secular realm, in which churches in concert with other voluntary organizations build the Kingdom of God. Today, Clyde Grubbs calls us a "radical universalizing movement"; it's a similar argument.
The creative essence of Unitarian Universalism has been its efforts to respond to humanism with this move: from the closed circle of salvation history of Christianity (God creates; Man falls; Jesus Comes; Church prepares; Jesus Comes Again; Man Saved.) to a much more open-ended process of development whose future is much more contingent. Instead of the Church preparing for the Second Coming, the Church learns and teaches and aims toward the Kingdom, or the Beloved Community.
There are a lot of issues that have to be addressed in this effort: how to worship, how to build our own communities, how to avoid insularity, how to escape our demographic isolation, how to define an appropriate spiritual maturity in this context, how to work pastorally etc. And UU's are hard at work on all these issues.
But what really matters, I think, is our purpose which is not to build churches and religious communities, but to take that religious impulse into the world: to humanize the global civilization.