I loved the music, the setting (postwar Vienna, shot on location - astonishing), cinematography, the realistic and credible - an intelligent and interesting - story (postwar cynicism amongst the defeated and black market dealings), the cast (outstanding - not just the obvious Orson Welles, his stunning turn as Harry Lime, but others - Joseph Cotton as the writer who was a gormless and clueless friend, Trevor Howard as the British officer with a quiet integrity, Alida Valli, as the betrayed love interest, and the others in a terrific cast.
Above all, I loved the fact that the movie had the courage of its convictions and was prepared to aim for the bitter sweet integrity of art, and allow for a downbeat ending, - when it became clear that Holly Didn't Get The Girl, as she walked out of the screen and out of his life - rather than seek to satisfy audiences with an expected 'happy' ending. Brilliant.