Transparency
While security is vital for voting equipment, it is just as important that the security can be demonstrated to the public. A secure system that people do not trust does nothing to enhance democracy.
This means that the system must be designed in a simple and comprehensible fashion, with rigorous testing to ensure the correct implementation of the security and functional requirements.
Lifecycle development practices must be applied so that the implementation of the system can be assured to be in accordance with its design.
Finally, the design must be transparent: design and test information must be available to independant parties for their review and analysis.