I've greatly enjoyed reading "The value of modern art," a 1948 lecture by Meyer Schapiro, published in the compilation Worldview in Painting: Art and Society. He wrote with a perspective that is rare for someone in the midst of major changes.
Schapiro attempted to analyze the formal qualities of the contemporary art of his period with the same eye that art historians train on art from their past. “In the same manner that we are able to distinguish the art of the Renaissance from the art of the Middle Ages, the art of Egypt from the art of Mesopotamia, by careful observation of the forms, so we discern in the immense diversity of modern art a broad constancy in its structure and expressive means” [138].
1) The artist leaves clear signs of the act of painting. “Hence, in modern painting, the touch or stroke is so very pronounced” [138]. The trace of the act of painting becomes an important part of the form of the painting.
2) “The modern painter treats the surface of the canvas as a concrete definite tangible ground, as an object in itself” rather than a transparent window into an image somehow beyond the canvas [139].
3) “The work is so designed or constructed that the composition though well ordered looks undesigned, independent of any a priori scheme. The artist does not aim at symmetry or a legible pattern” [139]. “The result is a constant interplay among chance, incompleteness, and the final order, completeness and rightness of elements” [140]. Schapiro presented this as a manifestation of the modern emphasis on individual creativity, self-expression and (re)invention.”
4) “in a modern painting, the artist preserves in various parts of the work the traces of the original instigating object or experience,” [140], rather than hiding the original impetus or pattern under a beautiful representation of a scene.
I cannot but agree with these generalizations of the formal qualities of mid-twentieth-century painting, and conclude that they represent the ascendency of the artist as individual auteur, visibly placing her/his creative ideas and process at the forefront.