Often described as one of the most influential Sci-Fi movies ever made, The Day the Earth Stood Still stands out from it's contemporaries for several reasons.
Firstly, it doesn't use aliens as an allegory for the threat of Soviet invasion (the Cold war having just started, and paranoia about possible communist infiltration of America being in full swing); rather, the aliens travel to earth to deliver a message to humanity (an early example of the Sci-Fi cliche, "I come in peace, take me to your leader"). In contrast, the film depicts the US military as almost institutionally aggressive - when the unidentified flying object makes a landing on the lawn outside the Capitol building, the army's first response is to surround it with a cordon of armed troops. Whilst this is arguably a realistic assessment of what would happen in this situation, it also serves to highlight the human tendancy to act aggressively against the unknown.
Secondly, it is a different type of story to its contemporaries. Contemporary movies were generally of the "monsters are outsiders" type, for example Invasion of the Body Snatchers or The Thing, reflecting American fears of the Soviet Union; The Day the Earth Stood Still stands out by being of the "monsters are insiders" type, where the "bad guys" are humans like us. The army take the role of "villain"; although the first shooting of the alien Klaatu (Michael Rennie) is arguably an accident, they subsequent imprison him in a hospital room and then hunt him down when he escapes before shooting him in the back when they catch up with him.
The visual effects used in the film were sophisticated for the time, with much use made of optical printing to insert "glows" to represent both the spaceship (while airborne) and the energy ray used by the robot Gort. However, the effects are primarily concentrated in the opening 10 minutes or so of the film, when the spacecraft lands and Gort first demonstrates his power in retaliation for the shooting of Klaatu.
Video 1: Gort Attacks
Most of the film is spent watching Klaatu interacting with human civilians in an attempt to learn more about the people he may end up destroying, especially widow Helen Benson (Patricia Neal) and her young son Bobby (Billy Gray). The interaction between Klaatu and Bobby is interesting, as Klaatu is rather an innocent, asking questions about things "adults" take for granted - for example, he is amazed at the fact that all of the people in Arlington National Cemetery died in wars, and has so little understanding of the value humans place on small, shiny objects that he offers Bobby a couple of priceless diamonds to cover the cost of visiting the cinema; at other times, he talks in an almost omniscient manner - explaining inertia in the manner of a university professor, or casually telling Bobby that "one day, he'll tell him about a type of train that doesn't need rails". This is helped by the high quality of the acting, especially Michael Rennie and Patricia Neal (Variety, 1951).
Video 2: The Day the Earth stood Still (1951) Part 3
Finally, the film makes extensive use of the Theremin in its soundtrack, along with electric organs and amplified string instruments, leading to it being described as one of the first films with a primarily electronic score (IMDb.com, 2011). The Theremins (there are 2) provide not only music, but also the sound effects for both the flying saucer and Gort; some credit this film as starting the association between the ethereal sounds of the Theremin and science-fiction films.
Video 3: The Day the Earth Stood Still: Dialogue-Free Opening
Video 1: The Day The Earth Stood Still 1951 Gort (2009) http://www.youtube.com/watch?v=Eun7SmpNr1I (Accessed on 29/10/2011)
Video 2: THE DAY THE EARTH STOOD STILL (1951) Part 3 (2009) http://www.youtube.com/watch?v=WqPj4-fiRfs (Accessed on 02/11/2011)
Video 3: Unusual Scoring of "The Day the Earth Stood Still" (1951) (2008) http://www.youtube.com/watch?v=TRc6f2oVdlw (Accessed on (05/11/2011)
Crowther, B (1951) The Day the Earth Stood Still http://movies.nytimes.com/movie/review?res=9A07EED61031E23BBC4152DFBF66838A649EDE&partner=Rotten%20Tomatoes (Accessed on 02/11/2011)
Errigo, A (2011) The Day The Earth Stood Still http://www.empireonline.com/reviews/reviewcomplete.asp?FID=132646 (Accessed on 28/10/2011)
IMDb.com, Inc. (2011) The Day the Earth Stood Still (1951) Trivia http://www.imdb.com/title/tt0043456/trivia (Accessed on 06/11/2011)
NF (2011) The Day the Earth Stood Still (1951) http://www.timeout.com/film/reviews/65054/the_day_the_earth_stood_still.html (Accessed on 05/11/2011)
Variety Staff (1950) The Day the Earth Stood Still http://www.variety.com/review/VE1117790275?refcatid=31 (Accessed on 01/11/2011)