Adiabatic electron transfer (ET) in the room-temperature ionic liquid 1-butyl-3-methyldicyanamide (BMI(+)DCA(-)) and in aprotic acetonitrile is studied with molecular dynamics (MD) computer simulation techniques using a model diatomic reaction complex. The influence of barrier crossing dynamics on ET kinetics is examined directly via constrained reaction coordinate MD, while the corresponding effect arising from activation and deactivation processes in the reactant and product states is analyzed with the aid of simulation results on solvation dynamics. The departure from the transition state theory (TST) rate constant caused by barrier crossing is found to be moderate and comparable in BMI(+)DCA(-) and acetonitrile despite a huge difference in their viscosity. A theoretical analysis shows that the Grote-Hynes theory yields a reasonable agreement with the MD results on barrier crossing in both solvents, whereas the Kramers theory fails completely in BMI(+)DCA(-). The influence of activation and deactivation dynamics on ET kinetics in BMI(+)DCA(-) varies markedly with reaction free energetics because of the biphasic nature of solvation dynamics, viz., ultrafast subpicosecond relaxation followed by slow subnanosecond decay. This indicates that dynamic factors controlling adiabatic ET in BMI(+)DCA(-) transition from barrier crossing to activation/deactivation as the barrier height for the forward and/or backward reaction decreases. This regime change of ET dynamics is accompanied by the breakdown of TST as the reaction becomes activation-limited in BMI(+)DCA(-). By contrast, activation and deactivation dynamics do not play a major role in acetonitrile.