Phenomenological similarities between the temperature dependence of the thermal and transport properties of supercooled liquids and certain crystalline fast-ion conductors are explored, using information from computer simulations and experiments. The similarities include non-Arrhenius temperature dependence of the transport coefficients, anomalies in the heat capacity, two-step relaxation of the time-correlation functions, and the connection between the mobility and entropy being described by the Adam-Gibbs equation. We use the detailed information available from computer simulation to examine the relationship between these observations and the number of defects in the system. Such defects may be unambiguously identified in a simple geometrical way from the underlying lattice in the superionic system; defects are often invoked in descriptions of supercooled liquid behavior but they are not easily defined in such a strongly disordered system.