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.
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 emotionThe 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.
Humanity will, just barely, survive the EMP and seek retributionThink 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 40,000 proud against the alien eco freaks. The aliens will never see it coming back to haunt them, either.
Klaatu's people trying to stop humanity's destructive ways was a Self-Fulfilling ProphecyInspired 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.
The movie is the setup for a Steam Punk futureThink 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-SpiralsWell, 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.
The series Revolution takes place in a alternate timeline of The Day The Earth Stood Still (original or remake).