Quick background, I watched Guardians of the Galaxy once again and after I finished I started thinking about any Easter Eggs or references that people/websites might have missed. Then I thought about the scene where Ronan confronts Drax on Knowhere. Surprisingly, (well for me at least) Drax gets his butt handed to him. But here is where the interesting part comes in, after Ronan has defeated Drax, he drags his body and throws him in the lime green Celestial liquid. I found it odd that Ronan went the extra-mile to do that having already knocked out Drax unconscious. I pondered upon that idea overnight, and in the morning I decided to find any information in regards to that...Here is what I came up with:
Drax the Destroyer
Arthur Douglas was a human whose family was attacked and killed by the supervillain Thanos. Needing a champion to combat Thanos, the being known as Kronos took Arthur's spirit and placed it in a powerful new body, and Drax the Destroyer was born.
In 2004, the character lost his flight and energy blasts, and a portion of his strength resilience. This version of the character played a role in the crossover comic book storylines "Annihilation" and Annihilation Conquest, and became a member of the relaunched Guardians of the Galaxy.
So based on the info above, we can assume Drax is based off of the 2004 character who lost a couple of abilities, but as far as him being originally human, that we do not know yet. Drax the Destroyer's body is specifically made to destroy Thanos, he is a living weapon, so it shouldn't surprise us that he was weak against and unable to defeat Ronan.
The Celestials existed near the birth of the universe and created life and death along with the multiverse.
New Celestials may be born by consuming the mass of an entire galaxy. Visiting Earth every few thousand years in groups called "Hosts", the groups is revealed to have been responsible for the creation of the Eternals and the Deviants, and, via genetic manipulation, the existence of superpowers in mutants.
Thanos is an Eteranal, that means the Celestials are his creators (keep that in mind). Knowhere is a criminal outpost built in the severed head of an ancient Celestial being, and everyone there mines for special artifacts or the valuable techno-organs (if that's an appropriate term).
So going back to the fight between Ronan and Drax: Drax is dunked into the Celestial juice. Groot poking his chest to release the liquid that made its way to Drax's lung suggests the Celestial juice is indeed in Drax's body. Therefore, I suggest, based on all of the evidence, that the Celestial liquid somehow activates or enhances Drax's abilities to make him invulnerable against Thanos. To clarify, since Thanos is an Eternal, a derivative of a Celestial, Drax (with Celestial chemicals) will be immune to Thanos; hence he will be able to defeat him.
I searched about my theory and I was surprised that a couple of people in forums picked that up as well. In fact, one person suggested that in addition to the Celestial liquid, the fact that Drax contained the Power Gem alongside his fellow Guardians will help him in defeating Thanos by absorbing the power of the Power Gem (that's a mouthful). Regardless, we will not know fully until Infinity Wars, if we're lucky, we will see some signs that either point toward or away from this theory in Guardians of the Galaxy Vol. 2.