By now, we're all used to the benevolent alien trope. The alien(s) come(s) to Earth, humans are suspicious, do human-type things like try to blow it/them up, and then we learn a valuable lesson about healing the world or at the very least something about friendship and loyalty. (There is, of course, the apathetic version: alien comes to Earth and is corrupted by our culture, but we'll leave that for another time.) Especially after the early 80s, when we seemed to be inundated by them. But in 1951, I can't imagine that a movie like The Day the Earth Stood Still came as anything but a surprise.
Coming shortly after World War II, in the midst of the Korean War, and in the early stages of the Cold War (not to mention the era of McCarthy/HUAC suspicion), The Day the Earth Stood Still showed a different future. The one that would happen if Mutually Assured Destruction were allowed to reach its logical fruition. The story seems familiar enough now that it's hardly a spoiler to say that in it, a man comes to Earth and tells humanity it's on the brink of destroying itself.
Looking back on it 58 years later, a few things strike me. The movie seems simplistic in its world view, both in terms of the dangers that beset humanity and the idea that all we need is to start talking again to ensure peace. It also seems “naive” (or blessedly optimistic, considering your view) about Mr. Carpenter's sudden appearance in the midst of the boarding house family and his easy association with little Bobby; Bobby's mother's boyfriend is jealous, but no one seems to suspect anything strange about a man who offers to watch a stranger's kid—that is, they don't suspect what we do now, watching it. Is that our problem, or theirs?
But what's saddest to me is the knowledge that this movie couldn't be made today. It wasn't, when the remake came out last year (which, for the record, I have not seen, though it has been described to me). One could argue that audiences are more sophisticated now, and in a sense we are—we demand more jargon and no longer accept “it's a powerful nuclear engine” as an explanation for anything, though modern films rarely say more despite their explanations and exposition. But most of this film involves a stranger coming to town, attempting to learn about its people, and talking while touring the D.C. sights. All the violence occurs off-screen. There's a chase that basically entails the army keeping tabs on the movements of a cab and reporting back to HQ. A strange man and a little boy talk about physics and Abe Lincoln.
The Day the Earth Stood Still is not my favorite film, but in this basic scenario, it is the film I want to see: the film about how a literal alien interacts with our culture and navigates his living situation. And then calls on his giant robot. I was surprised by how much I enjoyed it, despite its “simplicity.” It doesn't rely on adrenaline, and it makes a clear, if apparently obvious, point. Of course, the film started out as a point looking for a story, but I still admire its compactness.
Finally, while I knew the story and the politics going in, I was surprised by the fact that the “message” is so violent. Humanity must change its ways and become peaceful—by threat of force. Which begs the question: is this stance hypocritical, or merely Klaatu's practical way of dealing with an unenlightened species who understands nothing more?