Actually, knowing Arduino too well is far from necessary. They should be knowledgeable, yes, but anything more is just a bonus for the community and their efforts should be focussed as such ;-)
What is needed in a community moderator is the ability to communicate clearly and effectively, as well as have the patience to handle all types of members of the community respectfully. Also, to function impartially even when faced with difficult situations such as that may happen involving respected members, arguments, flame wars, etc.
Community moderators aren't referred to as janitors for nothing. There is a fair bit of work to be done behind the scenes, a lot of which may be fairly involved and taxing. The only reward for this effort is seeing the site grow and move towards graduation (which can be rather satisfying :) )
The only thing necessary is that the individual should be active and involved in the community through content generation, meta participation, chat availability, editing, reviewing and voting (close/reopen). Also, in general the individual should be helpful and guide new members of the community wherever needed.
Evangelising the community and being a good representative of Arduino.SE wherever the situation may so demand would be great too!
TL;DR Moderators should know Arduino.SE and we should know them; nothing else is needed.