What are the rules about using 'half of' with plural nouns?

You can use "half of" with plural nouns most effectively when you add the definite article:

Half of the users were women.

Half of the men were Canadian.

The reason for this is because you need to specify the group you are talking about. The definite article serves to limit the scope of the plural noun. It may require further limiting ("Half of the men studied were Canadian") but it doesn't sound awkward, because the reader or listener will presume that there is another qualifier modifying "Canadians" to which the definite article refers.

This is similar to the function performed by all and other qualifiers in your first set of examples. In that sense, in the first example you are declaring the scope to be the set that includes all films.

Half of all films are a waste of celluloid. [all provides the scope]

Half of users surveyed said they preferred the old product. [users surveyed provides the scope]

Half of the women here are your ex-girlfriends. [here provides the scope]

Half of Americans are uninsured. [Americans provides the scope, but maybe not enough].

There is no grammatical problem with saying:

Half of fruits are citrus.

It just sounds awkward, because the statement feels too general.