It's impossible for words to describe what horror is...
That's a line from Apocalypse Now, but it really applies to this film as well.
I read some of the post here and see some thoughtful, well planned out responses. Some about the actors, or the script, or how nothing really seems to happen. First off, I don't think a lot of younger viewers will immediately gravitate to this film. It was made before the "blockbuster" of Star Wars, where the senses are inundated with visceral delights. Sure there were big action movies before, but you get the idea. This movie wouldn't have been made if not for the success of Easy Rider, an anti establishment film that some how spoke to the movie going public of the time. I am trying to not use such generalizations as the word "generation" or the like. A time when the fabric of the American experience was changing for the young, when the nobility of the second world war seemed dated and at odds with the daily televised deaths of Vietnam. It's human nature to want more than you have, and this film dealt with that ennui.
I've heard Hellman say he wanted the acting to be like a canvas, so the audience could project whatever they wanted onto the characters. So in this way it is really a reflection of what you bring to the film. If you know in the pit of your stomach the longing and realization of emptiness, you may identify with it. If all you are use to is explosions and snappy smart assed responses, you may not. But when you get older you learn things about yourself and others, if you're lucky. This is also what Hellman described as French Noveau cinema, mascarading as a road movie. So everything is not laid out in paint by numbers fashion. It is understandable if you don't appreciate it, and after all it's just a movie, right?
That's just my take on one of my favorite movies. I may just be full of sh*t and don't know what I'm talking about, and that's o.k. too.