When looking at Formalism and Post-Modernism, especially when compared to Expressionism and Realism, one can really see major paradigms in how people think about and create art.
Formalism can be thought of as art for the mere sake of art. The content is secondary, if it is even considered at all. Post-Modernism is more of an amalgamation of styles, ideas, and intentions than one kind of unified theory. Both Formalism and Post-Modernism are kinds of responses and rejections of previous art movements where as Expressionism and Realism are less rejections and more different ways to interpret and express the same content.
For me, I think Formalism can be more interesting than Expressionism because rather than making me think about the content of the art, I am forced to think about the form. Though it can a bit abstract, like Peaches, Formalism gives a different view of what we see as art. It is the most basic form of art. Content without form is arguable impossible so it would be natural to examine the framework that one gives shape to content.
It is interesting to think, though, how different paintings would look like if interpreted by a different art paradigm. For instance, what would Starry Night look like if it was done in the formalist tradition? Blue field? Blue and yellow circles?