In addition to the point about sexual desires--something not openly talked about in the 19th century--Chopin creates a heroine who seems to abandon most of the attributes associated with an American female from the turn of the century. Edna dares to take up interests outside of the domestic realm such as swimming (this is, of course, in addition to her other exploits). She moves out of the house and "abandons" her children--a move that even today is considered more surprising and taboo for a woman that it is for a man. Most societies associate women with a nurturing spirit and believe it is unnatural when a woman gives up her parenting responsibilities. If we still have arguments in 2010 about whether a mother with small children should work outside of the home, imagine how scandalous Edna's choices would have been in 1899!
One other controversial factor is Edna's choice to commit suicide. While the Greeks and Shakespeare feature a myriad of suicides in their plays, Chopin's Edna would not have been viewed as a tragic heroine committing a noble act--at least not by an 1899 reader. Instead, many critics of the time characterized Edna as selfish and immoral and not a character whom they wanted influencing the minds of naive and innocent young ladies.
Chopin's The Awakening dared to suggest that women should be allowed the same sexual freedom that men have been allowed to have for, well, throughout history. Enough said?
The novel features a woman who refuses to fulfill societal expectations for women. In other words, she refuses to accept that being "barefoot and pregnant," as is said, is the only role a woman should be allowed to fulfill.
The novel features a woman who has a brain and acknowledges her sexual desires, and attempts to act on them. When the novel was published, Freud had already established the importance of sex in human existence. Though Freud's ideas have been heavily refined since, and though he, of course, went too far, the preeminence of sex in human motivation is a given. But these ideas were certainly not applied to women in Chopin's day. The novel dares to question America's puritanical ideas concerning sex.
Why this novel was condemned, labeled "poison," etc., is a no-brainer. It would have been a surprise if it weren't.