For the second time now I've found that a close vote has been cast for a question and I believe that the vote is based on a misunderstanding or misreading of the question. In such a case, I can only write a comment and hope that others will read it before joining the close vote. It would be better if I could vote against closing the question. I don't see a reason for the asymmetry of only having votes for closing, with a fixed quorum required for closure. In the limit of an infinite number of potential voters, an arbitrarily small fraction of them could close a question even if almost all potential voters are against.
As mentioned in the comments, the policy (roughly speaking) that has been adopted on MathOverflow is for a user to leave a comment on the question saying "I vote against closing," and for a subsequent user who wants to vote to close to leave a comment saying "I negate so-and-so's vote against closing."
Eventually SE did implement the ability to cast "Do not close" votes, though they do not work by directly cancelling votes to close.