The plasma parameters beta ⟨β⟩ and thermal diffusivity χE required for a self-sustained deuterium-tritium (D-T) reactor plasma may be written in terms of the geometry and technological requirements of the reactor. While there are, separately, constraints on each parameter, it is instructive to combine them in a single parameter ⟨β⟩/χE to characterize the reactor region. In a separate study of a generic toroidal magnetic fusion reactor it has been found that, for a fusion reactor to be competitive with alternative sources of electricity, it must have a fusion island weight of not more than half that of the Starfire and Mars reference reactors. A simple model, based upon this result, is used to determine the region of minimum ⟨β⟩/χE and the self-consistent physics and technology requirements for a reactor as a function of its plasma geometry. The requirements are compared with experimental achievements and theoretical predictions for the parameters ⟨β⟩, χE, and ⟨β⟩/χE, for a varietY of magnetic configurations – tokamak, stellarator, reversed-field pinch, bumpy torus, tandem mirror, and field-reversed theta pinch. It is shown that all these configurations are, 'classically', capable of reaching the attractive reactor region and that good progress is being made experimentally towards that goal.