I believe this should be listed as a comics minor key at minimum and my hope is that it gains the notoriety of major key for what it truly is.
This is the first appearance of Peter Quill in Earth-616, which should be precedence enough alone. See Marvel Fandom, or the complete marvel reading order for Earth-616 for reference.
Here Peter Quill interacts with Thanos and Gladiator in his first appearance in Earth-616. Prior to this the supporting characters from Earth-791 showed very little resemblance to the rest of the known marvel universe at the time.
Additionally, Thanos Vol. 1 is a bridge between two comic eras. Prior to this series and half way through, you have Jim Starlin who had provided readers with three decades of cosmic content to include most major cosmic events. Half way through this series you have the first author being passed the cosmic universe, who later helps develop the Guardians of the Galaxy. A cosmic team that dominated world culture for the last decade throughout the cinematic universe, comics, video games, toys and more. Additionally, as Keith Giffen takes over, he is the first to take on a major cosmic event post Jim Starlin, with Annihilation.
With this cosmic era being known as the era of reinventing past Marvel obscurities, you can pinpoint this issue as the issue that started the shift with the first appearance of Peter Quill in Earth-616. As the creation of the Guardians of the Galaxy was unknown at the time, you can only see the importance of this issue in retrospect. The same is said for most key Guardians of the Galaxy moments. The team formed around Peter Quill/Star-lord, so his first appearance is the moment an era was born.
Thus, why I strongly support this issue becoming a key book.