Of what significance is the fact that now his vantage point is “a great, high mountain”?

Two factors need to be noted regarding the significance of John's vantage point now being a great, high mountain.

First, from a high mountain one can see much more than from a hilltop, let alone from a plain. Thus, God took Moses to the top of Mount Nebo to see the Promised Land (Deuteronomy 34:1–3), and Satan took Jesus to a high mountain to see all the kingdoms of the world (Matthew 4:8). So we grasp the advantage of John’s God-given vantage point, particularly if John is to see a city of enormous dimensions.

Second, to view Babylon, John was carried into the wilderness (Revelation 17:3). The wilderness setting connected the woman John would see to the post-fall context in which those whom she would entice were living (Genesis 3:17–19; Revelation 12:14). In similar fashion, the high mountain connects the Bride whom John would see to the high point in the Old Testament city of Jerusalem, Mount Zion, namely, where David reigned as king and where God’s temple was eventually built. The Lamb, after all, is the King (Revelation 17:14) who comes to temple among the redeemed. The vantage point, in other words, reveals something about the nature of what John is to see.