Over chag I happened to be learning Mishna Brachot 9:6, which says "One may not behave frivolously opposite the east gate [of the Temple complex], which is in parallel with the Holy of Holies". That is to say, if one is directly east of the Temple and the various doors and curtains are open, they can see directly into the Temple, and this possibility requires them to behave with extra respect at all times, as they are in a sort of holy area. The gemara on this passage (Brachot 61b) clarifies that this rule only applies as far as "Tzofim" (another name for the Mount of Olives). Further east than this mountain, the terrain dips and the Temple is not visible, so no special respect is needed.
Of course that is the state of things now. But what if Zechariah's prophecy comes to pass? Then east of the Temple there would not be a mountain, but a canyon. The Temple would be visible not only in Jerusalem, but anywhere to the east (limited only by the earth's curvature, as we now know). This seems to be the symbolic meaning of Zechariah's miracle: in the future holiness will not be limited to Jerusalem, but extend to the rest of the world. This is the same message Zechariah talks about a few verses later: "Hashem will be king over the entire world; on that day Hashem will be one and His name one." (14:9)