I once recall way back when Spider-man II (Toby McGuire) came out reading an article talking about what made the webhead the most sucessful comicbook hero in the industry (yes, Superman was not always a reliable seller). What they concluded was the reason is that Spider-man was successful not because of the character, but because he had one of the best rogues galleries of any character. The article had the thesis that in any story, the character that is the most important is the antagonist, and that the truly great bad guys stand out because they think they are the heroes.
Every element of Thanos on his big screen debut was crafted to make a CGI ten foot tall alien something the audience could empathize with, even if they don't agree with him. From the degree of emotion on his face, to the way he spoke, Thanos was meant to be human. And why he was so effective was because we are so quick to attribute bad faith to people who do bad things, we do not stop to consider they may not be evil... they just think their doing good. Thanos' character is asking a very simple question: Is someone who is capable of doing so much evil incapable of loving others and thus irredeemable? And the film gives us the all too correct answer: No! Evil people are still capable of love.
Thanos is at his core, a good man, who does terrible things. A fitting final conflict for phase three of Marvel, which opens with a film where the antaganist isn't a villain at all. It's another hero. More importantly, it's another hero who believes he is right with as much conviction as the hero of the film.
Thanos isn't wholly wrong. In principal, he is right. Over-population is a real problem. It's a very natural problem. There are limiting factors on population growth in the wild. There is only so much water, or food, or any other necessary elements in a given environment and when those limits are allowed to reach, those competeing for the same resources are forced into conflict. And this is very much a human condition. There is finite land, finite water, finite comodities. Famine comes in this lack of wealth. Over abundence of predation leages to plagues. These are conditions largely out of our hands leading to the inevitable choice: Fight for me and mine, or roll over and die like the rest. The four horsemen of the apocalypse. The Biblical End of the World, summed up into one problem: There are too many consumers and not enough consumables. A very simple, very real, and very deadly solution man has faced since before history was recorded.
In reality, the logical answer to the suffering of all is of course, less consumers. This is something that does happen in the world all the time. Deer culls, the killing of a quota of deer in the enviroment so that they don't overcrowd and strip all the resources is common, as is the hunting and killing of invasive species. Australians have a hatred of both the rabbit and the cane toad to the point of psychosis because there are too many in their land. But it's something different when humans learn that they too, are reaching a thresh hold of population that cannot support. After all, one of the worst figures in the past century of human history was the who realized that the Germans needed more "living room" and went about a similar cull of the "problematic" population. And sadly, any environmental claim is dashed by the methods of selection... which is why the solution to human overpopulation is difficult because in our own competition with each other, we cannot help but question if the motives of someone we don't care about choosing out of some reason that is wrong.
Thanos thusly proposed a modified compromise solution to this problem: Overpopulation can only be corrected by culling some of the living. We agree that we cannot do this because no one should have control over who lives and dies. So we remove any control and leave it up to chance. Each person flips a coin, heads they live tails they die. The king of the mightiest nation will have no greater chance of living than the poorest man in the poorest nation. Nor the oppressor over the oppressed, not the man over the woman, the elder over the child, nor anyone with power over anyone else. This horriffies us for a number of reasons. First, the problem with the solution, for many, is not that it could be implement unjustly. It's that it is one person making the most life changing decision for everyone. And second, because as has been discussed, the other cold truth of reality is that in that moment, some people are in fact more important than others... but for reasons that are unbiased. Half of every thing will die. As the film shows, that's half of all pilots... both those on the ground and flying helicopters. Half of all doctors, including surgeons plying their craft, and half of all drivers, including those driving 18 wheelers in rush hour traffic. Thanos' plan is evil because it's impossible to kill off an even split of people. When potential evidence of bomb fuel was found at the Oklahoma City bomb investigation, the press asked the lead FBI explosives expert if the fuel was powerful enough to damage the building. The investigator, fed up with the questions so early in the investigation, explained that the power of a bomb's fuel was unimportant when compared to the location of the bomb and if that he was so inclined, he could bring down a 747 passenger plane with a cherry bomb in the exact right place. The reporters, curiosity getting the better of them, stopped asking about the very real bombing and started questioning him about how a thing that was more associated with juvenile delinquents could be the tool of terrorists. After much relenting, the investigator finally explained exactly where he would put a cherry bomb to completely destroy a sophisticated machine like the 747: Right under the pilot's ass.
Thanos is right in identifying the problem. No one argues "it's not too crowded" with him ever. Thanos fails because in his attempt to remove his or anyone's biased, he fails to consider that bias is the entire reason why it can never be properly solved. To the survivors of Thanos on that plane, the pilot's life is still more important than the entire passenger manifest. His motivation is noble... and he makes no mistake that he knows that he is not going to be praised for doing this. But he sees himself like Batman: He is the hero the people need, not the one they deserve.
This is why he is effective: not because he is a bad person, not because he didn't have all the pieces of the puzzle, not even because he miscalculated. It's because to his own mind, there is no other solution that will cause less pain. And we no he is well aware of pain. And that he's already in great pain.
And this has nothing to do with what Thanos says, but his own actions. The Red Skull says that the Soul Stone is unique because it displays wisdom and will only work for a fixed price: the love of the seeker's life. It's a lofty price, but it's a valuable judge for the soul stone, which has power over life and death. It will grant it's power to someone who seeks it not for their own benefit. If they are going to use it to return people to life, it will not be for their benefit. When the Avengers restore the lives, they still lost Natasha. They can make everyone else happy, but they cannot erase the grief of loss. If they intend to kill others, they will know the true scope of each death on everyone. And it is through his acquiring of the soul stone that the audice comes to see why Thanos is an effective villain. Because to everyone's horror, he's not a heartless monster. He does no what it means to truly love someone unconditionally. As much as Gamora and Nebula hate his guts and wish to see him dead... he still loves them. Thanos adopted his children, which to many is a sign of immense love... they aren't his own flesh and blood... they are strangers who are treated with all the respect and affection of a biological parent to their kids.
And when the snap comes at the end of the film, Thanos is granted a brief vision of a young Gamora, as innocent as the day she first entered his heart asking him if all his pain was worth it. To the father of six, who has just lost five children, he tearfully admits he is worth it.
This is as human as you get. It's always sad when someone dies, but its seen as a great tragedy for a parent to bury a child. Thanos is burying five in one day. And he is in terrible emotional pain. And despite all that suffering, all that grief, he still believes that his crusade is so important that he would be willing to lose so much to save people you do not know, but will always know will hate you and curse your name... well, it's hard to say that a character seen in that light would not seem heroic.
What Thanos did, was wrong. No one argues this point. But it's hard to say he is evil because as much as we know Thanos did something wrong, we still know his heart was in the right place. We still no Thanos was a good man, who did a very terrible thing. It is a description that, though the scope may not be as grand, no one in the audience can truly say they haven never experienced the feeling of being a good man who did a terrible thing. It's a very human condition.
Thanos was an effective and quotable character because he was handled as if he was a hero who did something wrong, not as a villain, who had lost his humanity.