At the end of the 19th century art started to become more subjective. For an impressionist the whole scene, an impression, and especially the light in it, was the subject. I myself like most impressionists. Expressionists expressed their feelings on the subject. Sometimes expressionistic images were quite overbearing, even to extent of being almost repulsive. Art has always been a way to express or vent out ones feelings, but this was the first time they were so aggressively visible! Vincent van Gogh is of course one of my favorites.
Along with photography, photorealism became irrelevant. Earlier it just meant that artist had nothing to say or s/he didn't know the subject or ignorant customer demanded such a thing, but now even that is done better by machines. This might have been one of the reasons why different art movements begun in the start of the 20th century. They were also a sort of a revolution against established canons of art like perspective and representativeness.
Artists started to make their expressions, feelings and thoughts the Main Subject. If the subject is abstract shouldn't the representation be such too? Cubism was one of the early forms of abstract that reduced the subject to more geometrical forms. As it also shows the subject from different angles at the same time it has always given me an impression of representing schizophrenia. Maybe it's an intentional reference to how rapidly world is nowadays changing and how many different faces we have to present to it during our lives?
Some of the artists experimented with form, color and new materials. In my opinion most such paintings, for example likes of the color and composition studies by Klein and Mondrian are `art for the artisans', reference material, generally not something I would call Art with a capital `A', because it's just too simplistic. I like the organic fractal beauty of the nature. Eye (I) likes subjects which provide different detail on different levels. In paintings, the environment of the painting and it's frame on the top level, differing hues and intensities in different forms on the next level, then images itself, real or abstract, their details, marks of the water, brush, pen or knife and the surface of paper, wood or canvas (media is part of the message). I don't understand this fascination of using only plain, boring, non-committing geometrical areas of uniform color. It just doesn't connect anywhere.
As pictures became more subjective, they were `understood' by fewer people. It's funny how people, who think or want themselves to appear culture-savvy, pretend to like something they don't `understand' or know how to relate to. I suspect that quite a part of the `modern art' is quickly done trash that has as little relevance for the `artist' as it has for the ignorance-guilty customers and critics. Nowadays doing simple abstracts couldn't be called very creative, just plain imitation. if it's `artiness' depends on what the artist tells us about it, how can it be `real' <grin> art?
But then, who cares if it's Art or not, as long as it has meaning to you, as an artist or as a viewer? And note that art, be it with a capitol `A' or not, isn't made for the sake of art critics, whether they are friends or strangers. Art is a form of self-expression!