This is a first thought, because it will make my next blog post -- about communion ware -- make more sense.
When we think about what it means to be "churchy" we're often -- but not exclusively -- talking about tastes and norms set by "the Ecclesiologists," meaning that medieval-focused, Romantic movement that overwhelmed the Church of England in the nineteenth century. For them, there was one correct style appropriate for Christian churches -- in a word, Gothic -- whether that meant fully expressed in stonework, or vernacularized into the carpenter style. Think of pointed stained-glass windows. Why did this style cross the Atlantic and denominational lines? The prevailing taste, keeping-up-with-the-Joneses and the perhaps nothing so pedestrian as who the church architects and suppiers were. (This isn't an original thought, and I've seen it in a few places, most recently in chapter two, "Capital Ideas: Building American Churches, 1750-1860." of James Hudnut-Beumler's In the Pursuit of the Almighty's Dollar.)
There are noteworthy examples of Gothic Unitarian and Universalist church buildings, but so as not to lose the point: the creation of a common vocabularly of taste that's hard to buck, save with variations, like the engrossed domestic style the Universalists seemed to favor, or the (later favored) colonial revival the Unitarians of Boston imposed on the Western churches who wanted financial support. And the less said about the post-war community centers hiding in their own private parksor forests -- the newer UU norm -- the better.
Of course, those days may be declining: not a particular style or fashion, but the ability of churches to chose the shape of their buildings at all. I can all to easily imagine borrowed, rented or shared spaces being a part of the survival strategies of Unitarian Universalist (and other) churches in the all-too-soon future. Consider how many newer congregations meet in office parks or retail space.
Is short, design will have to be expressed in ways other than the building, and without the influence of an eccumenical community of tastemakers. It will be interesting what we come up with, and if we appeal to older and more humble models.