That sounds like a loaded question, but I'll answer anyways.
I am not one who generally gets into analyzing the films I see, but I know when I see a good film.
1) Its just an all around great film: music, scenery, etc, etc.
2) It was one of those films that I felt you could really feel the angst and struggles of the characters.
3) I also thought the film did a good job of capturig the feeling of the period during which it was set.
(although I am sure that this point can be argued)