Filled with "thought-provoking social commentary and personal reflections", Agust D addresses themes of liberation—"from feeling a certain way...from letting the past and future...control us"—and freedom on D-Day.

Musically, the album features a diverse range of sounds and genres, including boom bap, "coats of Auto-Tune", alternative rock, and a piano sample from Japanese composer Ryuichi Sakamoto.

[2] The titular opening track "D-Day" features "distorted guitar instrumentals" that "build up for a few seconds before launching straight into an intense trap flow.

"[2] In English, Agust D raps the lyrics "Future's gonna be OK / OK, OK, look at the mirror and I see no pain" followed by "D-Day is coming, it's a fucking good day" in an "almost joyous" manner, "as if this feeling of contentedness is a revelation".

[8] Its title is a word play that references both the instrument and an interpretation of liberation specific to the song—"to advocate for freedom in a reality built on unspoken societal expectations and restrictions echoed in today's online culture".

Built on a "shadowy drill foundation", both rappers trade verses calling out the spread of misconceptions and misinformation, particularly in regard to themselves, online and in the media ("Millions of news coverage and gossip, the villain in this age of information").

[1] Agust D showcases his vocal abilities on the fourth track "Amygdala", a "guitar-laden, harmony-rich" song whose lyrics recall various painful moments throughout his lifetime, such as his mother's heart surgery, a hospital visit after his birth, an accident during his teenage years that resulted in a shoulder injury,[1] and his father's eventual diagnosis with liver cancer.

"[1] After questioning the purpose of his past suffering, the rapper concludes, "What didn't kill me only made me stronger / I'm blooming like a lotus flower now", suggesting that his trauma led "to a more resilient rebirth"[1] and emphasizing the album's underlying "connecting thread", the "idea of pushing forward through the difficult moments".

[10] Rhian Daly of NME gave the album a five-star rating, describing it as "rich and varied" and "inimitably Suga – or Agust D", with the rapper proving himself "an unstoppable, thought-provoking force".

Noting the change from anger and "those old barbed emotions" in many of the rapper's previous solo works, Daly wrote that on D-Day, Agust D instead "[set] himself free and stepp[ed] fully into the role of a wise social commentator and – at times – protector...wrapping up his trilogy in peak form."

Siroky opined that "the clarity [Suga] has when it comes to his art", derived from years of honing his skills as an artist and a musician, was what took the "record from good to great" and that even for non-Korean speaking listeners, sonically, "the story he set out to tell ends up linear and cohesive, remarkably so".