Andrew,

First let me say that your last point about understanding Jesus divinity as a product of the confrontation with Roman imperial theology is right on target. I agree with that 100%. That is EXACTLY the liberal view and Borg and Crossan have gone there and explained that in great detail in a number of books. Unlike you, I AM sure where that will lead because I've seen the theological perspectives of Borg and Crossan develop over that last 5-10 years as they have expanded that conversation in their books. I like where it leads because it produces a Christianity that has a much better chance of surviving in a post modern world.  I've seen how it has lead my faith and it is a good thing. It saved my faith, as I'm certain it can save the faith of so many that are crying out for this message but are blocked from hearing it.

I have no idea why you seem to shine favorable light onto one of the most important points in the liberal view of Jesus (his divinity as a product of narratives that paint a picture of Jesus in opposition to Roman Imperial Theology), then you seem to suggest that liberal theology is an "escape clause". I think you have a completely biased perspective of what liberal theology really is. There is some serious language and terminology decay and misinterpretation happening in our conversations.

Now to address your questions…

What exactly is lost if we take the sort of ‘liberal’ option that you recommend, and what is the impact of that loss on our claim to be in continuity with the agenda of Jesus of Nazareth? Or more simply: What does it mean to be ‘Christian’?

You like loaded questions don't you? Let's instead look at what is gained by liberal theology. 

The first thing that we gain is the survival of Christianity in post-enlightenment and now post-modern understandings of the universe. We gain a robust faith that no longer is dependent on a particular worldview as we wipe the message clean of any ancient understandings of the world and come away with a message that transcends culture.  Now by taking away those worldview and cultural underpinnings I don't suggest we throw them away.  We actually diagnose them in great detail to see how they have flavored the message, but the goal is to perpetuate the message rather than the worldview. 

Second, we gain a clear look at the agenda of Jesus of Nazareth rather than the agenda of those that followed and adapted his message to suit their needs.  The Jesus that is left for us is a much more robust and applicable character.  The deified Jesus of the later Christian traditions is increasingly irrelevant and Christianity should find its hope in the Jesus that lies beneath the symbolism and legends.  The symbolism should be kept and valued as the vehicle which preserved the message but recognized as metaphorical products of the narratives.  Keep the message and the vehicle but recognize where one starts and the other ends. That is sound Exegesis.

What is lost in liberal theology is the divisive competitive drive for a monopoly on truth and a divine pecking order that has plagued Christianity for centuries and stained its hands with blood.  Also lost is the need to create an artificial war between faith and science which faith is destined to loose.  We stand to loose a "God in the gaps" which is shrinking with each discovery. What we gain is a God whose immanent presence no longer lives in fear of what may be found next under a microscope, at the core of a black hole, or buried under the sand in a desert tomb. 

What does it mean to be Christian? To accept the vision of Jesus as my own and take my part in making that vision a reality.

When we are done making this paradigm shift, then the message seems very clear.  Jesus' goal was no different than Moses, Amos, Micah, and John the Baptist. He taught about the same just society as his leaders before him.  The difference with Jesus was that he resisted the urge to use the methods of Empire (violence, domination, and exclusion) to gain this justice.  His method was to acheive this justice through individual transformation from within and non-violent protest without.  Jesus says that the kingdom is not part of "this world" meaning that it is not something that can exist within the current domain of Roman Imperialism.  It is exists within us and only can be manifest fully outside the rule of any Empire (e.g. outside "his current world").  As long as any Empire or Imperialistic value system exists, then the kingdom cannot be an external reality fully experienced by all.  It is "at hand" in us when we accept the vision and it becomes reality as we slowly wipe out the values of Empire and create a kingdom on earth that mirrors the kingdom of Heaven.