After all these years practicing, listening to, writing about, and teaching music, the most fundamental unifying musical principal finally dawned on me. It happened while I was teaching a class. I heard myself say, while I was explaining the structure of the harpsichord cadenza in the Bach Brandenburg Concerto #5, the words "tension and release," and after class I realized that all music involves the successful or unsuccessful use of these two elements in proportion to one another.
Sonata form, for example, is all about tension and release. The repeat of an exposition is a release from tension (if set up properly by the performing musicians), a development section is all about setting up and building up the tension (harmonic and otherwise) that gives the recapitulation a feeling of release. Rondo form has the same buildup of tension and release, but it happens in different places in the movement. A successful multi-movement work "works" because of the sense of tension and release that happens between movements.
Bach often uses chromaticism to set up tension, and he relieves it in all sorts of ways: sometimes he does it gradually, sometimes he does it abruptly with tempo, sometimes he does it covertly, and sometimes he does it by resolving something cleverly (there are too many examples of this to name).
The observation and transmitting of musical tension is the responsibility of the performing musicians, but the composer is responsible for putting the necessary elements in the best order, and controlling the progression of the tension and release by using harmonic and melodic material, dynamics, orchestration, and tempo.
The ability to work successfully with the balance of tension and release is what divides the great composers from the good ones. An unsuccessful buildup of tension by a performing musician (or group of musicians) can make listening to even the most expertly-written piece unbearable, and an unsuccessful release of tension can make the most successful buildup of tension almost meaningless.
We have the responsibility as composers to be as vigilant as possible to balance the tension and release in our pieces, and as performing musicians we have the responsibility to follow the trajectory of the tension and release written into the music, and give performances that reflect a natural balance. It is much easier to do with "expert" music than it is to do with music that is less-well-written, and it's much easier to do with "common practice" music than with music that does not rely on harmony and melody to build up and release tension.