If it was just the culmination of all the other movies and the debut of Thanos it would have been good enough. But they didn't just introduce Thanos as some cartoonish, one dimensional villain. Like Steppenwolf in Justice League. You can argue against his logic, but he has a real motivation behind his actions, and comes across as a real person.
I was watching the Double Toasted review yesterday, and they mentioned how surprising it was that they had a character like that cry. Actually show a side other than generic tough guy. It reminds me of the Watchmen graphic novel, after Ozymandias succeeds and gives his monologue about his plan. He just sits there and you can see the enormity of what he's done hit him all at once. He was so busy making sure he succeeded, he never had time to really sit down and think about what success meant. I read that comic as a teen, and it was the first time I realized comics were capable of storytelling that really hit you on an emotional level:
He turns to his only intellectual equal, Dr. Manhattan, looking for validation that the choice he made was morally correct in the end. Only to be met with
there is no end, stripping away the moral relativity he's cloaked himself in, so he could live with his actions. Then the scene ends with a brooding Ozymandias left to consider his choices, and the nature of morality. Never thought a comic book movie would be capable of capturing that same kind of emotion.