What parts of speech can be the subject of a sentence?

I always told my students that the subject of a sentence had to be a noun or a pronoun. Pronouns didn't usually bother them, because the basic subject pronouns are so familiar to us: I, you, he, she, it, we, and they.

Concrete nouns were no problem: tree, desk, pencil.

Sometimes they had problems with abstract nouns: freedom, anger, seriousness, difference

To make it really simple, I told them the following: If it isn't a what or a who, it isn't a noun and can't be a subject.

For instance, freedom is what we want, anger is what we sometimes feel, seriousness is what we need when taking a test, and difference is what sometimes exists between two thing.

When we started studying dependent clauses, it still worked in finding subjects.

"Who will be speaking is a mystery." What is the mystery???

Who will be speaking is the...