Dictionary.com defines art as: "the quality, production, expression, or realm, according to aesthetic principles, of what is beautiful, appealing, or of more than ordinary significance." After pondering this question, logic suggests that what makes art good is then predicated on an individual's taste preference. If an individual can find beauty or appeal in the piece, then the piece has some form of artistic merit. But since artistic merit is then determined by individual taste, does that mean that a certain piece of art can be more "artistically good" than another piece of art based on the shear fact that more individuals like that piece than another?
Compare Da Vinci's Mona Lisa to Henry Darger's watercolors. Since the Mona Lisa is more well known than Darger's watercolors, is the Mona Lisa superior? I think some people would argue this point, that yes it is superior. But is that right? Again if we follow the definition of art, given to us by Dictionary.com, as long as the individual finds beauty or appeal in the piece, then it must "good" art. Hence the watercolors and the Mona Lisa are equals.
Not all pieces of art will have universal appeal to everyone. But isn't that the joy of art? That you can find beauty anywhere you look for it? By definition art should embrace diversity. The more I think about this question, the more I want to work on an audio documentary exploring art, and what makes art good.