We recently watched both the 2008 and 1951 versions of this story. It was interesting to watch them back-to-back and to have watched the newer version (I was not that fond of it) before the older one (I liked it).
Hopefully without spoiling anything for anyone, the summary of the story is: a strange object arrives on earth with 2 passengers, an alien "man" named Klaatu and a robot named Gort. Klaatu has come representing a federation of other life bearing planets to assess the potential for damage represented by man to the life bearing capacity of Earth. He warns that if Man cannot be trusted with responsibility to take care of the Earth, Man will be eliminated.
In the 1951 version, Klaatu is truly just an advance man, representing no real threat other than to Mankind's conscience. He spends some time getting to know the human race through a mother and son he meets at his rooming house. He blends in, and shows understanding, compassion and humanity, even a sense of humour. He demonstrates the potential power of the aliens by making the Earth stand still - bringing all mechanical devices to a halt for 30 minutes. In the end, Klaatu leaves having done little more than disturb the peace, with a warning about what might happen if Man doesn't smarten up and play nice with each other.
In the 2008 version, Klaatu is a bit more menancing, almost robot-like, which is understandable with the casting of Keanu Reeves in the role. There is a very strange scene at the beginning of the film, with Reeves as 1930's mountaineer discovering a strange glowing orb in the Himilayas that leaves him unconscious and with a strange mark on his hand; this is never explained in the story and so is truly pointless, serving only to allow a brief snicker at the poor make-up and scenery (for being in the mountains and a snow storm, he looked pretty warm, you could not see his breath at all, and the beard he wore was pretty ridiculous). In his later incarnation as Klaatu, he is much more at home, adopting the alien persona complete with automaton expressions. The rest is pretty unbelievable story of a him escaping and the dodging recapture by the entirety of the US Military (lead by Kathy Bates as the Secretary of Defense, sporting the wardrobe and hairdo that one might expect if Ivana Trump was Secretary of Defense. This was very odd casting.) aided only by Jennifer Connelly and her stepson and their Honda Civic. In the end, Klaatu and his GORT (now renamed with a typically ominous military acronym for Genetically Organized Robotic Technology) lay waste to much of the eastern US, only ceasing when Klaatu takes pity on the son and mom. However, his reprieve comes at a price - no more technology for Mankind, as he wipes out electricity and everything with a massive EMP. In this version, The Day the Earth Stood Still becomes the that last scene, and the Earth presumably stands still ever after as Man learns his lesson and takes better care of the planet. Unlikely.
I enjoyed the 1951 version more mostly because the alien was truly not a threatening presence. He lived among the citizens, learning about Mankind through the eyes of a young boy excited to show off his home town to a stranger. The film ends with the hope that Man will heed the warning. The 2008 version is almost like a sequel - Man did not listen to the first Klaatu and so the second one came on a clean-up mission. The idea that the other ships that accompany Keanu to Earth start to perform an ark-like function, collecting up all the species except Man, presumably to repopulate the Earth after Man's extinction; this was a nice little add-on to the story that did not get adequate coverage in the movie, which is too bad.
Hopefully the later movie will lead folks to rediscover the earlier film, as it is well worth watching. The 2008 version can be left standing still.
