Pratchett, like many other popular authors, have fallen prey to the classic controversy that his works are too simple for them to be considered as works of literature. Some argue that his prose is dull and unimaginative, that the conversational tones he utilizes to make his novels warm and inviting cause his novels to be poor literature. Yet on the other end of the spectrum there are those who view him as a literary genius: his melding plot into the humorist style, his incorporation of big ideas such as philosophy into a fantasy universe he creates and maintains throughout forty-one novels and several short stories, and his usage of language in the form of clever albeit silly jokes and puns.
His use of allusions and references also draw different criticisms. His references are usually much more modern than books usually considered as classic. He makes fun of tropes present in society, such as in Equal Rites where he explores the gender equality: despite women and men technically being equal in society, there are societal norms from both men and women that restrain women.
I've read Pratchett now: it's more entertainment than art – The Guardian
Jonathan Jones argues that Pratchett's prose is dull - while he explores interesting topics, his diction and language is not worth the praise often given to him.
Terry Pratchett’s Discworld Might Be The Highest Form of Literature on the Planet – Tor
Brandon Sanderson praises Pratchett's ability to merge plot, humor, allusion, and philosophy. Sanderson also mentions the characters within Discworld and how they are developed through their actions and dialogue throughout novels.
Shakespeare in Discworld: Witches, Fantasy, and Desire – JSTOR
Kristen Noone explores Pratchett's use of Shakespeare in his Discworld novels, most notably through the effect of which witches are used. Furthermore, she dives into the usage of Macbeth and Hamlet in the book Wyrd Sisters and the usage of a Mid-summer Night's Dream in Lords and Ladies.