In discussing an article I wrote a couple days ago regarding Degas' Little Dancer of Fourteen Years, a reader, who had known of the sculpture all her life, didn't understand why it was considered such a disturbing work when first exhibited in Paris in 1881. Today, I suppose, that's not a surprising question. Having not been there myself, and from second-hand sources, I'd have to say critics were disturbed first because it represented a sudden change in media for Degas, something with which the French at the time, and sometimes even we today, aren't always comfortable. Sculptors then were supposed to have studied years to master their craft, and the idea that some untrained jerk of a painter could create something so touching kind of rankled the critics. Second, he was displaying it in wax, which was considered merely one step in creating a bronze sculpture; so it was as if the work were being exhibited unfinished. And third, of course, was the "mixed media" element in adding clothes, hair, etc., to his figure. It was as if he were "cheating" in not sculpting them. So, really, it was a combination of things, the least of which was the figure itself, which, by all accounts, was considered quite lovely even then.
I think we should keep in mind that the art world was still pretty narrow-minded as to suitable sculpture media in 1881. There was stone and bronze, sometimes wood, and little else. Plaster, clay, and wax were considered merely a means to an end, and even wood was something you only carved. In fact, except for preliminary work in wax and clay, all sculpture at that time was created by means of the "subtractive" method (carving). Even more than Degas, Rodin suffered from these preconceptions. Especially concerning academic training, it was felt a sculptor had to have studied under a master for years. Rodin flunked the Academic entrance exam to become a sculptor three times, yet went on to master stone, wax, clay, and become the greatest sculptor of the nineteenth century.
As I suggested a few days ago in discussing the portrait bust today, stone carving is practically a lost art. Conversely, the idea of the "assemblage" was a twentieth century concept growing out of a painting tradition rather than sculpture. Picasso and others developed it as their collages became more and more three-dimensional. Today of course, in one way or another, most sculpture is created by means of the highly forgiving "additive" method, and the list of possible sculpture media grows almost daily.