The Voice of John the Baptist

Today we hear that most iconic of Advent voices, that of John the Baptist.  John is something of a load-bearing character in the arc of scripture -- he embodies the whole prophetic tradition of God's covenantal people, a tradition always pointing to God's promises and God acting.  What then does John's voice -- his demanding call for repentance and return -- mean for us in Advent in particular and the Christian life more generally?