The 20.4 MWh Superconducting Magnetic Energy Storage Engineering Test Model (SMES/ETM) will be the world's largest superconducting magnet - by nearly two orders of magnitude in stored energy. Its size, Lorentz loads and power delivery requirements dictate a number of fundamental design requirements that do not exist for other superconducting magnets. In addition, the significant capital investment in this prototype for commercial electric utility and defense application mandates that the SMES/ETM fully accomplish its primary mission of demonstrating large-scale SMES technology, while at the same time minimizing technical, schedule and cost risks. This paper presents and discusses nine key design decisions that are fundamental to meeting these challenging goals.