She had an eating disorder, she is seen throwing up or trying to throw up all throughout the movie.
The problem is we don't see a spiral of her going mad trying to attain perfection.
You know, Stephen King wasn't a fan of Kubrick's The Shining because Jack Torrance was just crazy from the beginning whereas there was a definite descent in the novel. This is how I felt about Black Swan.
From minute one the viewer knew she was crazy. To me, that isn't a story. I guess I was expecting some kind of twist and there isn't one.
There were a lot of 'too disgusting to watch' moments as well. Basically it was like it took Aronofsky 10 years to make a movie that was a lot like Requiem for a Dream, only far more inferior. The symbolism in this movie was so ridiculous! We see Nina in white, then shades of gray, and black; Lily in black with wings tattooed on her back! Talk about in-your-face. I prefer my symbolism to be a bit more subtle...when you are trying to be so
"deep."
At the end she was able to finish the dance because part of the mirror was still lodged inside; she didn't start to bleed profusely until she pulled it out.
Winona Ryder's part wasn't pointless, it was what Nina was striving for...the central theme of the movie.
I agree with a lot of what Sparx said - tried to be edgy, fell really short. A boring movie with really no redeeming qualities.
Try Requiem for a Dream. Or my personal favorite movie about obsession destroying people, "The Prestige."