I think what is important for lay people to understand is there are no obvious flaws in bitcoin. If they think they see an obvious flaw or some way in which it won't work, it is important that they understand that they don't understand it yet. This is difficult without insulting peoples intelligence, but I find if you let them 'save face' by explaining the underlying system is only really understood by cryptologists and mathematicians, they dont feel so dumb and resistant to something they don't get straight away. This gives them the time to absorb it.