I think admit defeat votes need to have 80-90% yes to pass.
This is known as a
supermajority, for your info.
IMO for some votes (eg this and draw etc) people who don't vote should be assumed to vote no.
This denies the ability to
abstain which is arguably an important third option. Yet in the absence of a
"None of the above" vote it's hard to tell whether people are merely neglecting to vote, in which case you're effectively arguing for a
double majority with the second criterion being that there is a representative
quorum in favour of the action.
I can't help but feel all of this is sidestepping the real issue. Absolute democracy isn't absolutely perfect (the referendum we recently had in the UK confirms that for me personally). I suggest all votes become merely consultative when an admin is present (possibly implementing a vote purely for admins when multiple admins are present). That way it's up to the admin to deal with the situation fairly, and if he fails then the players can protest by not playing there. This is pretty much the system we have already but a bit more formal so you don't get overzealous admins taking action without warning. Also this would mean no more cancel/pass votes which is pretty abrupt and intrusive, which will easily just end up annoying people. In any case, you could always have higher 'superadmins' who can act like they currently do and overrule everything, if necessary.
Otherwise just do the third "None of the above" option with a double majority criterion.