In Guardians of the Galaxy, the brief appearances of Thanos (Josh Brolin) backfire for Ronan the Acc (Lee Pace) as the main villain of the film. Ronan the Acc is a religious zealot who strictly enforces the Kree religion. He's also commonly regarded as a lesser MCU villain, despite being a major antagonist in the Guardians' movie entrance to the film side of the Marvel franchise.
Guardians of the Galaxy was the MCU's first big jaunt into the wider universe of Marvel, with only the opening scene taking place on Earth - or rather, Terra, as the film refers to it. It introduces audiences to the greater struggles occurring off-world and the driving forces behind them. One of these struggles is the 1,000-year war between the Kree Empire and the Nova-Corps of Xandar. Shortly before the start of the film, the two warring parties sign a peace treaty that enrages Ronan and leads him to seek Thanos' aid to destroy the Xandarian people.
Thanos' presence in Guardians of the Galaxy backfires and robs Ronan's role as the main villain, making him feel more like an errand boy than the antagonist of the film. The result is what many felt was a lackluster portrayal of Ronan the Acc as the main villain in Guardians of the Galaxy. Not only are Thanos' two adopted daughters Gamora (Zoe Saldana) and Nebula (Karen Gillan) sent to assist Ronan in retrieving the orb, but to oversee and make sure he accomplishes the task, suggesting the villain needs to be babysat in some capacity. Thanos also mockingly calls Ronan a "boy" and "pouty child" as a frustrated Ronan kills The Other in front of Thanos, undermining his intimidation factor and making it clear that Ronan is a figure to make fun of, not to fear. Even though Thanos isn't shown using his strength and skills, the few lines he does have demean Ronan. They make him seem more like a pawn in Thanos' search for the Infinity Stones than a proper villain.
Even in Ronan’s big moment of telling Thanos he no longer needs help destroying Xandar after discovering the orb contains an Infinity Stone, the scene still largely revolves around establishing Thanos' threat level. Korath (Djimon Hounsou) warns Ronan that he could not properly wield an Infinity Stone, mentioning that Thanos is the “most powerful being in the universe.” - said warning being proved true when Ronan takes the Power Stone and places it into his hammer because he can’t withstand the power of the stone on his own. The moment is supposed to show that Ronan doesn’t need Thanos to accomplish his own goals - but ultimately only establishes that Ronan is weaker than Thanos, making audiences want to learn more about how strong Thanos is instead of being invested in Ronan the Acc.
Perhaps the most blatant example of Thanos overshadowing Ronan is something Drax (Dave Bautista) says at the end of the film. Gamora tells him that his wife and child will rest well knowing he has avenged their death at the hands of Ronan as Drax is walking away with her. Drax responds: “Yes. Of course, Ronan was only a puppet. It’s really Thanos I need to kill.” Throughout the entire film, Drax’s main focus is on killing Ronan to get revenge for his wife and child’s death, as it's the main reason he s the Guardians in the first place. This throwaway line at the end of the film removes the importance of Drax killing Ronan, robbing both the character and actor of the satisfaction of getting a resolution for what initiates Drax’s character arc - especially given that Drax doesn't play a major role in the eventual killing of Thanos himself.
Thanos' future in the MCU. Ronan deserved to be a stronger villain, not merely Thanos’ errand boy as audiences got instead.
