Well, most famously the original and elegant use of timeshift and multiple, interwoven storylines. Add to that the foregrounding of conversational dialogue which allowed the film to have wonderfully nuanced characters and constant humour and many memorable scenes/lines. Not forgetting the brilliant use of music (not score) and filming techniques (like the special emulsion celluloid they used to give the diner scene its richness). But most IMPORTANTLY the textual (perhaps intertextual) and moral depth that underpins the films status as a masterpiece of cinema.