Leonard Bernstein wrote the music, and that was easily my favorite part of the movie. The dancers amazed me, and I can't imagine trying to come up with choreography to go with the crazy changes in time signatures Bernstein loved.
As for why I thought the movie was so-so, I agree with Roger Ebert's review from the re-release in 2004:
"So the dancing is remarkable, and several of the songs have proven themselves by becoming standards, and there are moments of startling power and truth. West Side Story remains a landmark of musical history. But if the drama had been as edgy as the choreography, if the lead performances had matched Moreno's (actress who played Anita, a supporting character) fierce concentration, if the gangs had been more dangerous and less like bad-boy Archies and Jugheads, if the ending had delivered on the pathos and tragedy of the original, there's no telling what might have resulted."
Big Names: Natalie Wood, Richard Beymer, Rita Moreno (who's won a Tony, Emmy, Grammy, and Oscar!)