Contrast. Gimmick or principle?
For sure, contrast is abused. I personally can't stand Apple's trademark long-short sentence structures: "We redesigned the iPad from the ground up. Three times." But undeniably, the contrast of sentence length works. It's a boast.
Contrast is especially abused in film scores, usually in two modes of imitation. I'm sure you've heard the tropes -- the crash of a Hans Zimmer score being stripped back to a single, swooning violin; the irony of violence set to some 80s pop song. These work on a basic Pavlovian level of filmmaking -- we've been conditioned by genuine works like Pulp Fiction and The Dark Knight to recognise these cues as tasteful, creative decisions. But they bastardise the beauty of contrast, or counterpoint.
Counterpoint is, I would argue, fundamental to any good work of art. It is about finding the balance in asymmetry, which requires being comfortable working with symmetries of greater complexities than simply squares. Counterpoint demands that you understand a piece of art as a whole, because contrast isn't just any kind of difference. In fact, it's probably better described as "difference in similarity", where A and B are really more like two aspects of a greater whole than two separate ideas. It implies a relationship. Contrast is really about drawing unexpected, but fitting, connections.
Trent Reznor is a master of this - listen to almost any Nine Inch Nails song from The Downward Spiral or The Fragile and you will see contrast in action. Take Hurt for example -- the whole structure is based around contrast between verses in minor, and choruses in major, and it even ends with a sudden crash of noise, as if the raw tenderness of the lyrics had suddenly collapsed in on themselves. It feels part of a whole, not random, and somehow inevitable.
And within filmmaking, outside of sound design, contrast is the entire principle underlying editing. You counterpoint a wide of a funeral with a close-up of the bereaved parent shedding a tear; you thus relate the universal with the personal. Or you do the opposite: cut from the action hero leaping away from an explosion to a wide of the fireball blooming over the city, relating the personal mission of our hero to the fate of the whole city.
Contrast is a very simple principle to get a hold on; but it is immensely powerful, and personal. Each brain makes its own connections, draws its own relationships. My contrast is different from your contrast. All have the ability to make thrilling new combinations of sound and image, as long as we challenge ourselves to look beyond the safety of tropes, and trust the weirdest, strangest, most true-to-us associations that bless our minds daily.