This movie is nothing special, or at least, it SHOULDN'T be. It's really a very standard storyline told through the lens of dance -- one of aging, the path not taken, friendships, and what it means to grow up.
The first thing that makes it exceptional is that it's a dance movie that has a good plot. It's got a reputation as one of the better dance movies, and it definitely deserves it. I find ballet rather tedious, and this movie didn't change my mind, but I really did enjoy the movie as a whole. Unlike nearly every other dance movie out there, this movie doesn't skimp on the writing and produce an insultingly simplistic script. There's no need for a dance movie to have a terrible plot, it just seems to be one of the really stupid conventions of the genre.
The other exceptional thing is that it is a story driven by women, that isn't a flighty piece of fluff. It's refreshing to see Hollywood acknowledge that women can be real characters with real motivations, and can be the focal point of a movie. Too often the "it won't sell to men" line is used as a rationale for casual misogyny among the movie studios. It was great to have a window to the 1970s when second-wave feminism was having a moment and it wasn't so outlandish for women to carry a movie.
I wish there were more films like this.