- In the 1950s movie, spaceman Klaatu comes to Earth because, with the advent of nuclear power and modern space travel, mankind is on the cusp of great developments that will lead us to the stars. However if we do go to the stars acting like warmongering monkeys, the stars will repay us by destroying mankind ruthlessly and completely. Klaatu then benignly demonstrates how powerless we are in the face of truly advanced civilizations like his and leaves it up to us to reform.
- In the recent film, Klaatu comes to Earth because we're screwing up our planet's ecology (just like his people once screwed up theirs). Habitable worlds are rare and Klaatu will destroy us utterly if we continue to abuse ours. Klaatu doesn't so much demonstrate his power as attack the mankind only to finally relent.
Among it's many problems is that if you have the ability to use nanites to fundamentally rearrange matter, as Keanu Klaatu does, you can basically do anything. Liveable worlds would not be scarce because you could terraform them. Formerly liveable worlds would not pose a problem either because you could fix that damage in short order.
Klaatu's concept of property and value is similarly broken. Earth is a precious commodity and we're misusing it. In some sort of warped communist morality, Klaatu is justified in killing mankind because others can use Earth better. Because intelligent life isn't a precious commodity I suppose.
Thankfully while the first movie is still held in high regard almost 60 years later, the second movie is being widely panned. There is justice in the world.
UPDATE: John C. Wright throws his well-written prose in the ring and links to reviews at Dirty Harry's Place.