Ratio of faculty positions to applicants
At least in the US, I can't think of very many fields where there are shortages of applicants for available faculty positions, especially in this day and age with the current economy.

Even when times were booming back in the mid-90's and in the 2000's, you'd still have dozens or even hundreds of applicants for any available faculty position. With the economy still somewhat depressed, you still have lots of people chasing after an even smaller number of positions than before. So I still think that in any viable field of academia, you'll still see applicants outnumbering vacancies. It's only in subdisciplines or departments that probably should not have been separated out in the first place that would end up having gluts of available positions.