His study of the development of Arabic poetry during the Umayyad Caliphate is still considered to be the most important reference book on the topic.
[1] His great work was his 100-volume History of Arabic Literature, a project that took 30 years to complete, and was an overview of all poetry, literary criticism and rhetorical studies from Pre-Islamic Arabia up to the post-Abbasid era.
[1] Daif caused minor shockwaves during the mid-20th century when he rediscovered the ancient linguistic tract of Ibn Maḍāʾ known as Refutation of the Grammarians.
Levantine writer and diplomat Shakib Arslan considered himself a defender of Daif against traditionalist criticisms, referring to himself as "the executioner of Shawqi's enemies.
Two of his grandchildren are also Cairo University Scholars; Prof. Mahmoud Elfiky, a professor of Pediatric Surgery and Dr. Sara Deif at the faculty of engineering.