Examples and Observations:
"Writing in about 100 B.C.E., the Greek grammarian Thrax, who invented the whole idea of the parts of speech, counted eight of them: adverbs, articles, conjunctions, nouns, participles, prepositions, pronouns, and verbs. The Romans had to drop articles (that is, a and the), since such words didn't exist in Latin, and added--hot damn!--interjections. The early English grammarians started out by adopting the Latin scheme, and it wasn't until Joseph Priestley's The Rudiments of English Grammar, published in 1761, that someone came up with the familiar baseball-team sized list that included adjectives and booted out participles for good.""A part of speech outside of the limitations of syntactic form is but a will o’ the wisp. For this reason no logical scheme of the parts of speech--their number, nature, and necessary confines--is of the slightest interest to the linguist."
(Edward Sapir, Language: An Introduction to the Study of Speech, 1921)
"When linguists began to look closely at English grammatical structure in the 1940s and 1950s, they encountered so many problems of identification and definition that the term part of speech soon fell out of favour, word class being introduced instead. Word classes are equivalent to parts of speech, but defined according to strict linguistic criteria."
(David Crystal, The Cambridge Encyclopedia of the English Language. Cambridge Univ. Press, 2003)
Open Classes: Nouns, Verbs, Adjectives, Adverbs
"The open parts-of-speech classes that may occur in a language are the classes of nouns, verbs, adjectives, and adverbs. Typically, each of these classes may be divided into a number of subclasses on the basis of certain distinctive grammatical properties. For example, the class of nouns in English may be divided into such subclasses as common and proper (on the basis of whether or not the nouns occur with articles like the: the girl vs. *the Mary), count and non-count (on the basis of whether or not they occur in the plural: chairs vs. *furnitures), etc. And the class of English verbs may be divided into such subclasses as transitive and intransitive (on the basis of occurrence with objects: enjoy it vs. *smile it), active and stative (on the basis of occurrence in the progressive: is studying vs. *is knowing), etc. Such subclasses are not ordinarily identified as distinct parts of speech, since there are in fact properties common to the members of the different subclasses, and since the label parts of speech is . . . traditionally reserved for 'major classes.'"
(Paul Schachter, "Parts-of-Speech Systems." Language Typology and Syntactic Description: Clause Structure, ed. by Timothy Shopen. Cambridge Univ. Press, 1985)
Closed Classes: Pronouns, Prepositions, Conjunctions, and Interjections
"Of the traditional parts of speech, . . . the pronoun, preposition, conjunction and interjection are closed [classes]. . . .
"Closed classes . . . are highly resistant to the addition of new members (though the term 'closed' should not be taken to imply that such expansion is strictly impossible). A topical illustration of the difficulty of adding to the closed classes is to be found in the failure to satisfy the widely perceived need for a singular personal pronoun to replace the he of examples like If any student wishes to take part in the seminar, he should consult his tutor: many people understandably find it offensive for he to be used for a non-specific member of a set containing both males and females. . . . [T]here is no morphological process appropriate for filling the gap and attempts to create a new simple stem (such as thon) have not been successful."