I vaguely recall seeing the original 1951 film The Day the Earth Stood Still, but it was many years ago so I had to use Wiki to refresh my memory in order to compare it with the 2008 remake which I saw recently. I learned something from this, which is that the 1951 film was based on the short story Farewell to the Master (1940) by Harry Bates, not one that I recall reading.
The 2008 remake sticks quite closely to the general plot and spirit of the original with some changes to the detail, partly to take advantage of modern CGI. To summarise (with spoilers – skip the rest of this paragraph if you don't want to know what happened): a huge, transparent, globe filled with swirling patterns arrives from space and lands in Central Park, New York. It is promptly surrounded by military forces plus some selected scientists. A humanoid figure emerges from the sphere but is shot and wounded; a giant robot then emerges and shuts down the power to the weapons. The humanoid (who turns out to be physically human once the outer covering is removed and bears a remarkable resemblance to Keanu Reeves) is taken away by the military from whom he is helped to escape by one of the scientists, Dr Helen Benson (Jennifer Connelly). He has a message from an association of civilisations in the galactic vicinity who have become worried about human aggressiveness: change, or be exterminated. He eventually decides that humanity will never change and sets in motion world-wide destruction of humanity and its works (the opportunity for some novel CGI), before Benson puts in a final appeal for a reprieve.
It clearly would not be fair to assess the 2008 film by the same standards as the 1951 version. The original was a ground-breaking film which has rightly become a classic; the new version merely updates the story for a modern audience. Ignoring the original for the moment and judging the 2008 version on its merits, it is not a bad SF thriller and is painless to watch, although not particularly gripping. Perhaps the worst aspect is the comic-book military action, including one point of detail which particularly jarred with me: someone in the production team was obviously impressed by the name "Sidewinder" since they used Sidewinder missiles to attack the globe on two occasions. In fact, this is a short-range air-to-air missile with a very small warhead, which is just about the least suitable missile in the US inventory for attacking such a target.
Overall, not a film worth making a point of watching, but bearable. Probably more rewarding to spend the time watching the 1951 original, which although obviously dated is a genuine landmark.