Steve Reich, in his 1968 essay "Music as a Gradual Process" declared:
I am interested in perceptible processes. I want to be able to hear the process happening throughout the sounding music.
Mr. Reich's great early works, like Clapping Music, Come Out, and It's Gonna Rain exemplify this desire in telling and expressive ways. The means (slowly phase shifting phrases) and the poetic ends are inseparable.
What results is, I think, a kind of transparency wherein the expressive intent of the composer comes through regardless of the means employed, be they complex and/or simple. One is tempted to think of this as a matter of texture--the thinner the texture, the more transparent the music. There's a relationship there, but counter-examples spring immediately to mind, like Feldman's For Samuel Beckett, Reich's Come Out (especially near the end), and the micro-polyphony of Ligeti's Atmospheres.
Complex works can be transparent, as well. Carter's Fifth Quartet, for instance, Boulez' Repons, and the late Beethoven quartets, to name a few examples.
Just as complexity for its own sake can easily lose its transparency and become mere complication, so too can simplicity become simple-mindedness and the transparency turn into nothingness.