A "Progressive Christian" should definitely look closely at the Old Testament before assuming that it's irrelevant - as many unfortunately do.  A lot of human progress that we take for granted today is chronicled in its pages.  It's a good thing that you have an open mind and are asking questions! 

To understand the OT, it's important to get into the mindset of what early human societies were like at the time the characters in the books were having their adventures.  Without this background, it is too easy to discount nearly all of it as archaic craziness which can lead to overlooking the deep truths and startling human development contained therein. 

I recommend an excellent book called The Gifts of the Jews by Thomas Cahill.  It is a fantastic read (you seriously won't be able to put it down).  From it you may get a better appreciation of just how radical  the main ideas presented in the Jewish bible really were.  You will learn that with the Jews, humankind first emerged out of the dark ages of religion - which often entailed a belief in many gods, a sense of no past or future, and no recognition of the sacredness of individual life.   All reality was just a never-ending series of seasons, turns of the wheel, births and deaths, gods acting out their dramas in the heavens, everything repeating itself over and over again with no hope for or even awareness that there was a possibility of anything new. There was no need to write down any history because such a concept was by nature irrelevant, and ultimately meaningless. 

Something radical happened when Abraham had the unique thought of doing something new and different: starting off on a journey with his family - a journey which, when written down, would become the beginning of all recorded human history. (I'm not talking about the beginning of writing, business records, myths, or narratives. Just history.)  This history tells the story of a people (and even mentions traditional "nobodies" like women, by name!) who have a personal relationship with a Power that they see as being a co-creator with them in life.  A Power that can give meaning to what's happened in the past, encourages ordinary people to plan and work in faith for a better future, and through a deep-felt communion brings peace in the current moment.

Furthermore, YHWH handing down the "Ten Words," which, like it or not, have become the foundation for all civilized societies, was unprecedented.  Such a thing had never been seen before and nothing has been seen since that can match the universality of truths contained in such a brief list of principles for civilized human life, coming from such an ultimate authority. Humanity went from blindly sacrificing virgins and kings, worshipping animals, statues, and/or heavenly bodies, and holding sex rituals to appease gods they saw as having all-out control over everyone and everything, to believing that a Force outside themselves wanted the people themselves to chart their own destiny, and with a sense of justice and concern for the weak and needy even!   Suddenly life was full of possibility and hope, whereas before there wasn't even the sense that anything in the future could be hoped for; such things were only for the gods and their pleasure.

Many added details seem brutal to the extreme (like stoning people for adultery), but put in the context of a primitive society attempting to prevent the chaos and injustice that would be created with out-of-wedlock births, clan fights, etc., it becomes more understandable (even if not acceptable) for them to have tried to live within certain ethical/safety/moral guidelines.  The overall goal was, after all, harmony and happiness for the people (especially the poor and downtrodden) which would please God. 

In some ways, it has probably harmed humanity to "wake up" to the ideas of past and future, because with them come guilt about the past and anxiety about the future, and all the stress that goes with such feelings.  But, if stepping out of the superstitious cyclical moon-god reality meant that for the first time humanity began to look at itself from a moral standpoint, it has probably been for the best. 

For Christians, even though the OT is a treasure of history and insights, all of its commandments and directives need to be held against Jesus' teachings.  Jesus' message, timeless and fiercely compassionate, wins out every time if there is a discrepancy.