I just finished reading The Prophetic Imagination by Walter Brueggmann. This is one of the best books that I have ever read. I highly recommend people to read this book.
Walter says that the prophet job is to do two things: first to proclaim that the system that runs the world is dead - that it cannot (and never could) keep the promises that it gave. The system that we live in does not bring life. Thinking about our world right now we live in a system that states that by consuming things we will be happy. Most of us live very well. Better off then most of the people in the rest of the world (and of the people in the past). We truly do live like Kings and Queens. However there is depression. Buying things has not made us happy it has made us jealous. Walter states that the role of the prophet is to announce this through grief and mourning. The system we exist in is dead.
The second thing that a prophet is to is to energize the alternate community that Yahweh is bringing forth. The prophet does just speak about death he speaks about life. Yes, the system we live in cannot give life but Yahweh can. There is a hope for newness because Yahweh is free from the control of people (this includes the church) and he will be faithful. This newness comes to the ones who are oppressed (the outcasts). Walter says that this is because they are the ones who are willing to accept that the old system is dead.
Well that is a very brief summary of the book. I highly recommend to everyone to read this book.