Despite its limitations, magnetohydrodynamic theory remains the best possibility for a predictive framework for the large-scale dynamics of a magnetized plasma. The mathematical structure is very similar to that for Navier-Stokes fluids, and indeed contains fluid dynamics as a special case. Fluid dynamics would not have gotten very far without understanding the crucial role played by dimensionless numbers (such as the Reynolds number) in classifying its regimes of different kinds of behavior. The situation seems to be much the same in magnetohydrodynamics. In particular, the Hartmann number, familiar in the theory of MHD power generation, seems to be the crucial number describing the onset of MHD activity in voltage-driven dissipative equilibria that model such confinement devices as tokamaks. Stability thresholds are calculable and the supercritical behavior above those thresholds may be quantitatively compared with the numerical computations of Shan et al. (1991, 1993).