Minimalism began as a general reaction to post-WWII modernism, atonality, and integral serialism. As such, there are certain traits that one find in minimalist pieces, but the technique is not uniform (just as there are many ways of composing 12-tone music, for example).
Characteristics of minimalism:
- diatonic/modal pitch content
- use of repetition
- short rhythmic cells/patterns
- steady pulse
- the use of process
- often long duration (for works)
Other characteristics appear more in a particular composer.
Phasing is a technique most often used by Steve Reich. Reich will repeat a pattern in multiple instruments, then shift the relationship between the instruments to create changing cross-patterns of accents.
Terry Riley uses drones, ostinatos, and indeterminacy in his minimalist compositions. As Riley is influenced by free jazz, his use of indeterminacy in performance can be seen as relating to jazz.
It should be noted that not all minimalist pieces are tonal/diatonic. Some do not even use pitched material. Diatonicism and tonality are just much more commonly used in minimalism than atonality.