The beauty and complexity of Mozart’s final concerto is not in drama and surprise, but rather in grammar. My teacher Leon Kirchner was fond of saying that Mozart was the most intellectual composer and indeed this is a piece that bears out that impression, both for its complex phrase structure and its complex form.
The opening is so deceptively simple—strings playing a childlike oscillating accompaniment under a simple stereotypical Mozart phrase that the woodwinds answer with a brief loud fanfare descending in an arpeggio. No big deal—that is, until you notice something is off. Phrases in the classical era are supposed to neatly balance as 4 measures+4 measures. That doesn’t seem to happen here. For one thing, we have a measure of introduction before the melody. Does that count as the beginning of the group of four or not? Maybe not, because the first phrase of the tune that follows is 4 measures. But then again, maybe so, because the short wind fanfare that follows the tune seems to enter two beats too early! That seems to throw the violins off when they start the second phrase of the tune. They also enter two beats early, but then seem to notice and skip ahead a measure, finishing their phrase in just 3 measures instead of 4. The winds then come again two beats early and the strings repeat the same “mistake.” What a mess! The entirety of the opening is now 13 measures instead of 16! The complication is that one measure wind fanfare. It seems to be throwing a monkey wrench into what otherwise would be a simple 4 + 4 tune. If Mozart had simply attached the fanfare to the tune, then the phrases would each be five measures instead of four. Mozart's tricky solution is to cut off the tune two beats early, then insert the fanfare. The effect preserves the balance of 4+4 phrase structure, while still keeping everything off balance by seeming to begin each new phrase on a weak beat!
This is what I mean by Mozart’s intellect. This took considerable structural imagination to make a conventional tune sound fresh and surprising.