I find Mozart (and Haydn) so interesting because they never went very far outside what an ordinary listener can enjoy even on first hearing. But at the same time, I never get the sense that they are pandering or condescending to the audience. If Haydn chooses to start a symphony with a drum roll, he is not pandering to anyone, he is just doing something different, for fun.
I am listening my way through all the Mozart piano concertos right now and there are some wonderful examples of great aesthetic results achieved through the simplest of means. Take for example the slow movement to Mozart's Piano Concerto in G major, K. 453. As is usual, the slow movement is in the subdominant, C major in this case. Here is the first page of the score:
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This is a very simple passage. But there are some interesting things going on. First of all, this is a five-measure phrase. Five-measure phrases are odd and outside the norm. In this tempo, the phrase should be four measures long. What he does is expand the last measure with a delayed resolution in the second violin, followed by an ornamented descent from the suspended 4th (of the dominant G major harmony) to the root of the chord. The harmonic movement is from I to V via the vii°7 of V. But what makes it really extraordinary is the fermata. As soon as the phrase completes, the music just ... stops. This is really both strange and effective because it is so unusual. On a more subtle level, this enigmatic little phrase is like the antecedent part of a period--but we don't get a consequent.
The next thing that happens is that the movement seems to start again, as if that first little phrase had never happened. What we get now is a stretto-like passage for the winds with an entirely new theme (the only common element is the little chromatic appoggiaturas) that goes on, seamlessly, for a new phrase totaling thirteen measures (where eight or sixteen would be the norm). Mozart does this kind of thing! When the piano finally enters, it repeats that enigmatic five-measure phrase exactly, fermata and all. But what follows is quite different as the music immediately moves to G minor. Though the winds' stretto-like theme soon comes back. Mozart is constantly giving us something new, but at the same time he is blending it with things we have already heard and expect to return. He is showing us lovely melodies and harmonies, but giving them strikingly different rhythmic settings.
That enigmatic five-measure phrase keeps coming back when you least expect it. He gives it to the winds to introduce a solo section for the piano a bit later on:
That fermata, which at first seemed so arbitrary, now proves useful to set up a new section. For example, the phrase is later given to the piano, in the original key:
and used to set up a new section in E flat major:
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It is functioning as a kind of "anti-rondo"! I say this because a rondo theme, typically used in final movements, is usually a rollicking kind of dance theme and between it are tricky episodes that take us far afield. The episodes are structured so as to bring back the rondo theme in unexpected ways. But here, this little phrase, which always ends with that hanging in the air feeling, is used to set up all sorts of different episodes. Its function is to introduce and it does it by halting the musical flow. I can't think of another phrase used in this way. But of course, that is one reason Mozart is great: he kept coming up with new ideas.
But here is something else to notice: while very original, this is not a complex or perplexing idea. It is something any listener can hear and enjoy on first hearing. It is not esoteric, though it is certainly unusual.
Now let's listen to the movement: