You've probably played the "my favorite subject heading" game when geeking out with librarian friends. Here's some additional fuel in case you've run out of zingers.
The Open Library takes the LC subject headings and breaks them apart at the subfield level into subjects, persons, places, genres, and times. It also includes some BISAC headings retrieved from Amazon, so the subject list is not "pure." The separate subject entries obtained are similar to, but not the same as, OCLC's FAST headings, and look much like some facets that appear in library catalogs.
The Open Library database currently holds about 24 million records for books (at least partially de-duped). In a recent dump of subjects, the total number of different subjects came out as 1,278,539. Of those, 336,638 were of the "topical" variety, that is either a 650 $a or a 65X $x. The top 25 are as follows:
212822 Politics and government
192968 History and criticism
123838 Law and legislation
95555 Juvenile literature
93364 Description and travel
90866 Economic conditions
84787 Criticism and interpretation
71468 Social life and customs
70926 Social conditions
69205 Private Bills
69191 Private bills
63301 World War, 1939-1945
60235 Foreign relations
55460 Study and teaching
I find it interesting that with the exception of "World War, 1939-1945" these appear to have the function of qualifiers, and I'm thinking that it would be interesting to contrast the $a and $x terms. My guess is that these are $x, but that not all $x are of this nature.
Of the subfields, 164,342 appear only once in the database. These are a great source of interesting an unusual headings, including "Social aspects of adzes" and "Deer as pets." In fact, the "Social aspects...." tail is so amusing that I have made a file of those with a count of 1.
The full file of topical subjects is 8 megabytes, but can probably yield innumerable hours of library cocktail hour amusement. (text in format "count - tab - subject") I will also look into names, organizations, places and times as subjects.