One could argue that a defining characteristic of the difference between the medieval and modern world view is the decline of faith and rise first of reason and then of subjectivity. 

Medieval or "Gothic" art was, to a large degree, religious, rooted in a firm set of convictions. Rather than realistically representing individual impressions of some element of the world, instead, medieval art strove to convey eternal truths, functioning anagogically to lead the viewer's soul from earthly to heavenly beauty. Abbot Suger's treatise on the Gothic cathedral, for example, argues for use of neo-Platonic theories of harmony in designing the spacing of pillars and other elements of church architecture so that the physical church imitates the underlying divine order of the universe. 

The modern era, starting with the Renaissance, marks a shift to a more empirical world view, with a greater emphasis on individual opinion and experience, but still has a sense of an objective reality reflected by art. In contemporary art, those assumptions of objectivity and transcendence have disappeared, being replaced by a view almost solipsistic. This leads to an increased focus on the individual artist as the subject of a personality cult. Absent the notion of some external objective standard, art criticism becomes either grounded in rhetorical communities or increasing a matter of individual opinions (as in Amazon reviews, crowd-sourcing, or judging works solely by price or popularity).