It's a great movie. Both story and acting. that's why.
The reason it's such a classic is because of the story. It has been cited as a major influence in many Westerns, and many other movies as well. If you watch a movie about a group of people who fight against a larger force, chances are it was influenced by the Seven Samurai.
I took a Japanese cinema a few years back and I've watched the movie in class. The professor was a huge Kurosawa buff and we pretty much watched all the classics. I'll have to admit I did doze off a bit watching seven samurai(it was a morning class) but the high score has merit.
Kurosawa's cinematic style is probably why people are giving the movie a great score. For example various undertones and themes are emphasized though the the composition of certain scenes and there is tons of symbolism everywhere. Although to see such things, real critical observation or multiple watchings of the film are needed.
Kurosawa introduced a number of cinematic techniques or types of "shots" that are commonplace in films now, but at the time revolutionary. Seven Samurai's greatness is evident as film directors today look up to Kurosawa, for example George Lucas. Things like the side swipe scene transition that Lucas has used in his films were originally seen in Seven Samurai.
I guess to really see the genius in Seven Samurai you'd have to have some background knowledge on films and their history and to not just watch for entertainment value but to see the atristry in Kurosawa's directing.
