Thank you for your compliments.
That's a good question you ask. I can't say I know for sure, but I have my own understanding of the answer. You make a very good point that Jesus' first coming was, in a sense, great and awesome.
Looking deeper into the Hebrew might help us.
My NASB Bible gives this translation of Malachi 4:5-6...
"Behold, I am going to send you Elijah the prophet before the coming of the great and [i]terrible[/i] day of the LORD. And he will restore the hearts of the fathers to their children, and the hearts of the children to their fathers, lest I come and smite the land with a curse."
The Hebrew word for "great" in verse 5 is "Gadowl" (#1419). This word speaks of "great (in any sense), exceeding, far, high, long, [i]loud, mighty, noble, proud thing[/i]." It also comes from the Hebrew root "Gadal" (#1431) which speaks of "to twist, to make large (in various senses as in body, mind, [i]estimate or honor, also in pride[/i]): [i]advance, boast, bring up, exceed, magnify, proudly, tower[/i]."
In contemplating this word, I could see how you might understand it as speaking about Jesus' first coming. But I see Jesus' first coming more in the sense of a humble, quiet coming. Yes, it accomplished great and mighty things, but in a humble and quiet manner. Jesus didn't receive honor and praise from all men. The ideas expressed in the Hebrew of loud, high, mighty, proud, noble, honorable, etc. don't seem to really fit His first coming, as I see it. Though I certainly see your point and don't think it's necessarily wrong. I'll explain more in a moment.
The Hebrew word for "terrible" in verse 5 is "Yare" (#3372). This word speaks of "to be afraid, to fear, revere; to be feared; to be dreadful; to be reverenced; to terrify, make afraid, holy respect." My OT Word Study says, "There are two main types of fear described by [i]yare[/i]: (a)the emotion and intellectual anticipation of harm, what one feels may go wrong for him; (b) a very positive feeling of awe or reverence for God, which may be expressed in piety or formal worship."
In contemplating this word, Jesus' first coming seems even further removed from matching it's description. I suppose you could kind of match it up if you really tried, but it doesn't seem to fit real well.
Instead of being loud, high, esteemed, revered, honored, and dreaded; He was despised and forsaken of men. We did not esteem Him. He was crushed, scourged, oppressed afflicted, yet He did not open His mouth. Like a lamb is led to the slaughter, and like a sheep that is silent before its shearers, so He did not open His mouth. (see Isaiah 53)
But He will have His day! His day of honor and glory is yet to come. When every knee will bow and every tongue will confess that He is Lord. All will revere Him in that day. I believe that will be the complete fulfillment of "the great and terrible day of the Lord."
But I wonder, how profound and deep is this truth? Could both of His comings be found in it? Could Malachi be referring to both of His comings?