I have the feeling that with the growing influx of questions, an increasing number of questions that are clear closing candidates (i.e. it is totally obvious and undisputed that they belong closed under one of the close reasons) slip through the net because they do not attract enough attention, and thus do not accumulate enough close votes, especially on weekends.
However, unless you start collecting examples manually, it is impossible to verify this because unsuccessful close votes can't be queried in the data explorer.
I think this is an important metric, and is going to become even more important in the future.
Can the data structure be changed to store these unsuccessful closing attempts in some form, so they can be queried in the data explorer?
Ideally, the following data would be queryable:
Number of total closevotes on a question
The types of the votes (not sure how this would have to be done properly - normalizing votes into a separate table sounds awfully complicated)
Note: The point of this is the ability to get stats, not necessarily for finding questions that may be closeworthy! However, Waffles points out that there are only about 10,000 questions that have more than one closevote that has withered away, so having these stats wouldn't really help much. I'm accepting his answer for that reason.