I know art in that I know the theory, and I am a painter so I can apply it. I know what is aesthetically pleasing and can differentiate quality based on usage of color, value, light, and composition. So, I may not be the best at telling what is or isn't art, but I can certainly tell you how good or bad it is.
As to what is or isn't art, I have my opinions. Art itself is not subjective, but the interpretation of it is. It is human nature to see things different based on perception. The quality of technique and application however, can be measured. It is the quality of art that concerns me, which is why I like Gelsey Kirkland more than almost all other prima ballerinas. She has artistry. Robert Bateman has artistry. There is true artistry when something is so compellingly and convincingly portrayed that it leaves room for only one interpretation, the intended one.
So, art can be judged in two ways, technically, or emotionally. One is objective, one is not.
Wolfedge has a point. The intention of the artist makes a big difference, but only if you know it. What made M.C. Escher's staircases so compelling was that he obviously knew what he was doing. A vague intention helps nothing and only leads to those strange, verbose interpretations of fine art speculators that make the common folk wonder what the man was smoking to see it that way.
The real problem is when you take two paintings, for example, of equal technical quality. What is it that makes one better than the other, more artistic? The true artist, somehow, gets his intention and thoughts across. You can sense what he was thinking or intending when he painted it. The better the artist does this, the better an artist he is.
Art is appreciated in two major ways, and the artist usually only accomplishes one. He's really good if he does both. One, through sheer technical brilliance. Robert Bateman does this. Two, through feeling. Edvard Munch does this. God is the first one to come to mind who consistently does both.
Because art does this, and good art is compelling, it can be dangerous to society. It can also be beautiful and uplifting for society. It depends, and that is the subjectivity.