The Day the Earth Stood Still/WMG

== WMG regarding the original ==

Klaatu is also a robot.
What seems more likely: that a race from another planet evolved to be virtually identical to the humans of Earth; or that they sent to Earth two robots, one only vaguely humanoid but indestructible and with a great deal of firepower, the other vulnerable but indistinguishable from an intelligent, clean-cut, good-natured yet serious human -- in other words, an ideal ambassador, and one much more likely to be listened to by humans than a sac of amorphous protoplasm, or a five-foot armour-plated leech, or whatever the aliens really look like?
 * In a loose sense, the short story "Farewell to the Master" that inspired the movie's plot involves Gnut being Klaatu's creator and not the other way round. It could be that, in that other planet, robots are "living beings" and living beings are "robots", conceptually speaking.
 * The new movie seems to support this; in it, Klaatu is played by an actual robot, Keanu Reeves.
 * He could, in fact, be a bioloid representative of the Scarran Imperium.

Or, he's a clone.
It seems sensible that the aliens might not be able to create a robot that looks convincingly human, but growing their own altered human from scratch would explain Klaatu's vulnerability. They also x-ray him early on in the film and find out his anatomy is EXACTLY the same as a human's, so a robot seems unlikely but a clone very likely.
 * Again, refer to the short story's twist.

Gort is the real boss.
This is the final reveal of the original short story, "Farewell to the Master".

Klaatu is a future human.
Klaatu doesn't just look human, he is human. Humanity survives the cold war because of Klaatu's interaction, and eventually builds those neat saucerships, robocops, and temporary revival machines. They go to space, but never find the aliens that rescued them from the brink of nuclear war all those years ago. They develop time travel, to try and observe the events of the movie in more detail, but discover that Klaatu's ship came out of nowhere, and disappeared back into nowhere at the end of the movie. They send back a saucership with a robocop and cause the events of the movie to happen, so that they'll be able to make them happen. It's all a big Temporal Paradox!

The message passed on to Gort is the magic words from Army of Darkness and the whole movie was a Xanatos Gambit to summon the deadites to kill all the humans
Gort and Klaatu were sent to destroy the humans before they made interstellar contact on their own.
 * Ah, but those words would have STOPPED the Deadites. It was Ash saying them wrong that led to the plague getting worse. Obviously, aliens were trying to save us from the deadite hordes until a certain S-Mart employee shows up.

Gort is a God Warrior
From the manga of Nausicaa of the Valley of the Wind, more of his kind appear; and they nuke humanity once they show they can't stop warring with each other.

Klaatu Barada Nikto...
...means "Klaatu lives not." There was a deadline by which Klaatu was supposed to tell Gort not to destroy Earth, and the only way to stop him after he gets going is with a distress call from Klaatu.
 * Not, a pre-programmed codeword for Klaatu forgives? So that when Klaatu dies, someone who was told the codeword could tell a robot to prevent vengeance with the destruction of Earth?

Gort is a trans-human.
Instead of an actual robot, Gort comes from a civilization who upgraded themselves through cybernetics. Klaatu is a cloned meat-puppet he created to interact with humans because he didn't think they were advanced enough to understand the concept.
 * So you're saying Gort is a Cyberman?! Cybus or Mondas?
 * Only less evil.
 * You have to ask? Mondas. Look at his outfit.

The people "back home" don't know about this whole "go around and put races on trial" thing; if they do, they just try to pretend they don't.
Okay, so let's get this straight. You send an Emissary to A-bomb possessing planets and tell them to either stay out of space, get rid of their nukes and join this galactic super-UN, or get blasted to a cinder. Sounds strangely like CIA covert operations, in a way. Who cares if we've overthrown legitimately elected governments, just so long as they don't help our enemies or threaten our superiority!
 * So, kind of like Special Circumstances in The Culture?

Klaatu's people are enslaved to robots like Gort.
His people regret the decision to leave the policing to the robots, but the robots by nature prevent any going back on that decision. While the robots aren't opressing them, their presence is stifling. If the humans would have had enough firepower to actually defeat Gort upon arrival on earth, Klaatu would have been asking for Earth's help to free his civilization from the robots. This is why Klaatu advanced towards the military with a weapon-like device - he was specifically trying to get exactly the reaction he got. The device may have even been a weapon humans could have used to defeat Gort, and Klaatu was lying about its true nature after it was broken. Because the humans didn't/couldn't defeat Gort Klaatu had to just go on about his job like normal, and hope the next planet he is sent to can save them.

== WMG regarding the remake ==

In the remake, Klaatu is an angel.
When he's asked what his original form was, he says "it would only frighten you". But he knows that the people he's talking to are trained scientists, and they would hardly be frightened of an enormous bug or amoeba or whatever. So his original form was something that really would frighten a bunch of rational, agnostic scientists - an angel.
 * You know, that makes FAR too much sense. It can't be correct. ;)
 * There are non-supernatural alternatives which would still scare the scientists. For example, a huge Goatse.
 * Uhh huh, so he's really an Angel? Then where were the DoD's equivalents of the EVAs?
 * The Department of Defense used Reapers and other Unmanned Air Vehicles (those giant remote-controlled planes) for that. The control schema for a UAV is very like that for a mecha; the big difference, besides form-factor, is that they are unmanned. Even this DoD sees that as an advantage.
 * Agnostic is not god-phobic, right? (If you meant atheistic, the answer is no.) And rational definitely is not. Either way, would it not make more sense to have them pick up the angel and start interrogating him while they attempt to discover how his body/brain works/is made of?
 * Finding an actual, provable supernatural being of any kind would fascinate any rational, agnostic scientist. Not that most scientists are either rational or agnostic-- most belong to a religion and they're about as rational on average as any group of human beings.

In the remake, Klaatu is an Eldritch Abomination.
Klaatu is not afraid that he will frighten the scientists by telling them what he is; rather, he is afraid that he will completely destroy their world view and drive them all insane.

Klaatu and the observer go insane after being subjected to human emotion
The observer quickly realized that humanity needed to be destroyed if the Terran ecosystem was to survive, but after enough time spent as a human, he also decides that they're wonderful creatures and would rather die than live in a universe without them. Klaatu reacts similarly, but because he gets more intense emotion over a shorter time, he ends up caring about Jacob personally instead of Humanity as a whole. That leaves him unable to kill Jacob despite knowing it to be necessary, and he decides instead to Take a Third Option no matter how unlikely it is to lead to a good outcome for his own people. In both cases, a sufficiently dispassionate and genre blind observer would call them suicidally insane.

The remake is the beginning of the punishment Klaatu promised if humanity failed to shape-up.
Two Words: Keanu Reeves. Soon, Keanu Reeves will be starring in all of our movies, and this will cause a black hole of despair and destruction that will consume the Earth.
 * Until Bill and Ted arrive from the past to save us all with their message of lighthearted humor and silliness!

Humanity will, just barely, survive the EMP and seek retribution
Think about it. The EMP will cause such widespread global disarray that millions, if not billions, will die. We'd be lucky to retain any level of modern technology, society, and democratic government afterwards. This is going to make people very afraid of the alien federation... and very angry at them. What's more, the only people who know that Klaatu spared us are the scientist woman and her son, who probably will have credibility problems considering they aided and abetted Klaatu.

Fast forward a few centuries after reverse engineering the dead Gort cloud remains, and humanity will start a jihad to make Warhammer 40000 proud against the alien eco freaks. The aliens will never see it coming back to haunt them, either. "Klaatu2: You disgusting machine things are still less than one million times as advanced as us.

Humans: It does not matter. We have devoured the mass of thousands of stellar systems. We outnumber you one hundred trillion to one. Zerg Rush! Zerg Rush! Zerg Rush!"


 * Considering that the EMP would have the least affect on those that don't use electricity in the first place (pre-industrial cultures, Amish, other assorted Luddites, etc.) those groups would both be in the best position to dominate the society that results and have the least reason to seek revenge. If FTL can be reverse-engineered within a few generations, before memory of the attempted genocide fades, a splinter group could try to start up the jihad described above, but Earth as a whole would be unlikely to go along with it willingly. Of course, if the aliens decide to just ignore humanity for several thousand years, the above scenario could still happen without Earth involved.

Klaatu's people trying to stop humanity's destructive ways was a Self-Fulfilling Prophecy
Inspired by the above trope, if humanity had been left to its own devices they would have eventually become a better race and taken care of their earth, and would eventually expand peacefully into space. But now that Klaatu and his kind nearly wiped out humanity, we have a reason to continue being warlike; we need to protect ourselves from them and any other aliens who want to destroy us for not meeting their standards. Since no one knows if and when they will be back, humanity primes itself for defence, and when Klaatu comes back ... there's a very nasty surprise waiting. "Human: "Take this message back to you people." *BOOM!*"


 * It gets worse. The environment's carrying capacity would probably be unable to support even half of our population without modern tecnology. Entire ecosystems will go extinct while we attempt to rebuild because hunger comes before environmentalism.

The movie is the setup for a Steampunk future
Think about it. They EMP everything. But that wouldn't affect things powered by steam. When we rebuild, we will have steam powered tanks, computers, etc.

They should, indeed, give up or blow us all up. It's not like this is gonna be better for the non-humans on the planet than the pre-attack status quo.

The humans will begin to develop biological superweapons that are unEMPable.
They'll eventually end up as something like Prototype times the Tyranids. And since the aliens think anything natural is soooo much better than technology, they wouldn't do more than smile and nod.

Klaatu and his ilk are Anti-Spirals
Well, they're the proto-Anti-Spirals. See, after the horrifying cataclysm that Klaatu caused, Humanity unlocks Spiral Power, and begins the process of becoming a grand empire. The Anti-Spirals don't like that, so they do... what they do; impose a stasis on themselves, attack Humanity, and (temporarily) win.