Andrew Brown has posted an outline of the basis of Unitarianism. It is true that this is a very important part of our heritage and continuing tradition, but I would have to add the results of interfaith dialogue, namely an openness to insights from other religious traditions. The dialogue with other traditions began early on, and appears even in the writings of Servetus, who referred to "Hermetic" texts, according to Earl Morse Wilbur, historian of Unitarianism. The pantheist tendency (which Andrew Brown embraces) and the nature-loving tendency both began fairly early on, as I explored in my article Pagan tendencies in Unitarianism. Another very important factor was the encounter with Rammohun Roy and the continuing relationship with the Brahmo Samaj.
I have always thought that a tradition's theology (whatever it is) cannot exist meaningfully if it behaves as if there were no other possible understandings of the world, but must explain and celebrate the existence of other religions - as liberal traditions generally do. For instance, when a liberal polytheistic religion meets another tradition, it adds some of the gods, goddesses and heroes of that tradition to its own pantheon, or assumes them to be equivalents; when a liberal monotheistic tradition meets another tradition, it assumes that they are worshipping a different manifestation of the same Ultimate Reality.
Doing this does not have to undermine the coherence of the original tradition; in fact it should strengthen it, because it takes the other tradition as a confirmation that the Divine is everywhere and speaks to all humanity.
So yes, there are certain values and ideas which are outside the Unitarian tradition - for instance, I imagine that a hard polytheist would be most uncomfortable within it, as would political conservatives. But there is a broad range of ways in which we can interpret the Bible, and cross-reference it with other great spiritual texts in order to elucidate its meaning, as John Andrew Storey did. Because the Bible is part of our culture, we cannot understand our legal and moral system unless we engage with it (even if we want to change the system, it is important to understand what it is based on). And the Bible is too important a text to be left to conservatives.
I very much liked Stephen Lingwood's outline of Unitarianism and how it brings about spiritual transformation. This emphasises practices and values rather than beliefs.