Who is the “Angel of the Lord”/”Angel of God”?
Why did the Angel of the Lord progressively appear less and less in the Old Testament?
– I didn’t know this was the case, but good points are made.
What characterises all the appearances of the Angel of God?
What was Christ doing before He came to this world, taking human flesh?
– at times God’s messenger, God’s manifestation, and the giver of grace in time of needs.

Dr David Murray (who blogs here) gave this address, “A Biography of the Christian’s Savior”:
http://vimeo.com/moogaloop.swf?clip_id=11665447&server=vimeo.com&show_title=0&show_byline=0&show_portrait=0&color=00ADEF&fullscreen=1

Summary:

“A Biography of the Christian’s Savior” will show that Christ’s biography did not begin with Matthew 1:1. Rather, it will show that the eternal Son of God was present and active throughout the whole Old Testament as well. We will look especially at his appearances as the Angel of the Lord.