This is one of those arguments that nobody can win because both sides are wrong and both sides are right. When you get right down to it, art is entirely subjective. There is no way to give an absolute evaluation of any type of art. However, because a large number of people can agree on what is good and what is bad, criteria has been established, and art can be valued against the criteria to come up with an assessment of its value based on these categories. While it is not perfect, studying art enough to come up with definable characteristics does lend credence to opinions rather than simply, "I may not know art, but I know what I like." It may be true, but it's impossible for the speaker to explain what it means to anybody else. People who define art as purely subjective might dismiss any kind of attempt to grade art as impossible or wrong. And those who believe art has to live up to at least a certain standard would argue that criteria are necessary for any type of discussion about art. What's probably true is that the answer is somewhere in between. As a certain user here wonderfully misquoted recently: "Opinions are like assholes. Everybody is one."