What's so hard about secure electronic voting?

Tonight's talk by David Jefferson at UC Berkeley sounds fascinating:

The security, privacy, reliability, verifiability, and legal requirements for electronic voting are profoundly subtle and complex, making it among the richest and most fascinating security problems known. The requirements, for example, are much stronger than those for electronic commerce. The depth of the problem arises from the subtle interplay of security, privacy, and verifiability requirements for elections that have no analog in other applications, and from the vital requirement that elections be secure against insider attacks (by programmers and election officials) in addition to secure against voter misconduct.