Simplicity: The more complex a system, the more vulnerable it is. The reasons for this are myriad, but basically, a complex system has more things that can go wrong. There are more potential vulnerabilities in a large office building than in a single-family house, for example. The antidote for this is simplicity. Of course, many systems are naturally complex, but the simpler a system can be designed, the more secure it is likely to be.
