The nature of online communities is such that you simply won't have
access to the information that would be required to make an informed
judgement about an applicant. You can't require details of their
Wikimedia activities since they could simply deny being a Wikimedian
(and I believe we are agreed that we don't want to restrict membership
to Wikimedians only). If you know of a reason why you should reject an
application you would be reckless to ignore it, but there can be no
reasonable expectation on you to do any research into applicants,
since such research is impossible.
The risk of lots of people joining to manipulate board elections is
very real - if we had the default staggered board resignations it
would be much reduced, since there would never be a majority of the
board being elected at one time (barring early
resignations/deaths/etc.). Unfortunately, I didn't think of that when
I was trying to persuade you against having everyone resign at once,
perhaps if I had, I would have been successful... Anyway, what's done
is done, the best way to mitigate the risk now is to recruit as many
good members as possible - the more good members we have the more bad
members will need to sneak in in order to gain a majority. Recruitment
will need to be one of the top priorities for the board after the AGM
(it should be a priority for the current board too, but not a top one
- handling setup is top).