We report on numerical simulations of simple models of athermal, bidisperse, soft-core, massive disks in two dimensions, as a function of packing fraction phi, inelasticity of collisions as measured by a parameter Q, and applied uniform shear strain rate (gamma) over dot. Our particles have contact interactions consisting of normally directed elastic repulsion and viscous dissipation, as well as tangentially directed viscous dissipation, but no interparticle Coulombic friction. Mapping the phase diagram in the (phi, Q) plane for small (gamma) over dot, we find a sharp first-order rheological phase transition from a region with Bagnoldian rheology to a region with Newtonian rheology, and show that the system is always Newtonian at jamming. We consider the rotational motion of particles and demonstrate the crucial importance that the coupling between rotational and translational degrees of freedom has on the phase structure at small Q (strongly inelastic collisions). At small Q, we show that, upon increasing (gamma) over dot, the sharp Bagnoldian-to-Newtonian transition becomes a coexistence region of finite width in the (phi,(gamma) over dot) plane, with coexisting Bagnoldian and Newtonian shear bands. Crossing this coexistence region by increasing (gamma) over dot at fixed phi, we find that discontinuous shear thickening can result if (gamma) over dot is varied too rapidly for the system to relax to the shear-banded steady state corresponding to the instantaneous value of (gamma) over dot.