This, despite not being the original - it began life as a play in Central Europe - has weathered the several incarnations that followed (MGM's own remake with period songs In The Good Old Summertime, the Broadway show She Loves Me, even the excellent theatre revival in Paris a couple of years ago) and remains the definitive version and the one they all have to beat. Several previous commenters have identified the contributing factors that make it so successful and memorable not least being the prevailing fashion in 30s and 40s Hollywood for lavishing attention and detail on ensemble playing rather than just two leads as so often happens today - try, for example, removing Ugarte, Ferrari, Renault etc from Casablanca and yes you'd still have Rick and Ilsa and Viktor Lazslo but they'd just be frosting without the rich cake mixture below. Jimmy Stewart and Maggie Sullavan WERE both ideal and irreplaceable leads but how much brighter they shine when their performances are reflected in those of Frank Morgan, Felix Bressart, Joseph Schildkraut and Andy Hardy's Sara Haden and this is BEFORE we factor in that Lubitsch 'touch'. Okay, maybe they WERE a tad more naive, innocent even, in that Jurassic Age but how many genuine film lovers, sated with scatology, screwing and in-your-face sex, turn back to those days of Stories, Style, Slickness and Skill and wallow in great movies like this one. By far the best thing about this technological age is not CSI but DVD that can at one and the same time make these classics available to nostalgics and show the Matrix freaks how the big boys used to do it.