What defines the "kingdom" fungi?

It is a certainty that the tree of life can be broken up into large units of biodiversity that span such great evolutionary distances that one could call them kingdoms. However, if these units are to have any meaning, they must represent radiations of diverse groups underpinned by unique shared derived adaptations that drove the evolutionary success of each group. Without such evolutionary characters, these classifications are merely abstract concepts or arbitrarily demarcated lineages. Even the five kingdoms (commonly called plants, animals, fungi, protists, and prokaryotes) that gained popular acceptance from Whittaker (1) were abstractly defined and nonmonophyletic. These kingdoms were defined based on cellular organization (prokaryote or eukaryote), multicellularity (protist or "higher" eukaryote), and trophic level (producer [plants], consumer [animals], or decomposer [fungi]). Somewhat more phylogenetically sound definitions (2) have replaced these classifications, but the names, defining characters, and number of kingdoms are still not settled (e.g., 3, 4).