What Foscas said. Additionally, it was one of the first shows to tell such a dark tale--to end act 1 with two dead bodies. It incorporated a lot of operatic techniques. The demanding show gave way to the new requirement of performers who could act, sing AND dance. Also it's reliance on young talent was influential. I mean, WSS was voted the most influential musical of all time by playbill.
I know what you meant Munk, but since the staging/direction and action is so important, I still maintain that is why it's not read.
BUT...just one minute...I'll give you it's not the best libretto, but score?! The complexity of the music, the way the score tells the story, the mix of Latin rhythms with romantic with tension with chaos, I mean I could go song by song, but I don't understand how one can't consider WSS as one of the greatest scores of all time. Wait, you included score as part of why it's not the best, but then indicated it part of its brilliance. Whatever. Score's amazing.
Besides, Robbins' direction/choreography has to be taken into account when you think WSS. Whether or not you consider that part of "paper" it's definitive of WSS.
"If there was a Mount Rushmore for Broadway scores, "West Side Story" would be front and center. It snaps, it crackles it pops! It surges with a roar, its energy and sheer life undiminished by the years" - NYPost reviewer Elisabeth Vincentelli