euangelion :: what is the Gospel?

In an attempt to "redeem" this blog and blogging as an exercise, I'd like to begin a series of discussions concerning the gospel. Tonight I'd like to start with the question, "What is the gospel?" It's a fair question, I think. We claim to be a people committed to the proclamation of the gospel and yet I have claimed, and I am not alone, that the church has at a minimum lost part the gospel in our practice and proclamation. In my opinion, what we hear being taught today is often gospelish and yet not the gospel in its totality. In Mark 1:14-15 Jesus said, 

14Now after John was arrested, Jesus came into Galilee, proclaiming the gospel of God, 15and saying, "The time is fulfilled, and the kingdom of God is at hand; repent and believe in the gospel."

What was Mark referring to when he claimed that Jesus was proclaiming the "gospel of God"? If we have lost the gospel, what in this description of "the gospel" have we failed to communicate?