Values Dissonance: Nowadays the book is barely as raunchy as your average YA novel (see G-Rated Sex), but back when it was published it was considered too inappropriate. (Chopin's short story "The Storm" is quite a lot more explicit, but it also wasn't published during her lifetime.)
On a second note, Edna's suicide is appearing to become interpreted as more selfish by today's students than in years past.
Vindicated by History: The novel was divisive amongst contemporary reviewers (although contrary to Common Knowledge, it did receive some favourable contemporary reviews; the St. Louis Post-Dispatch wrote, "There may be many opinions touching other aspects of The Awakening, but all must concede its flawless art.... Complete mastery is apparent on every page", while the New York Times wrote, "The author has a clever way of managing a difficult subject, and wisely tempers the emotional elements found in the situation"), but by The '60s it was being canonised as a lost classic. Though this is slowly being Inverted, as many readers today notice more problems with the book, and its reputaion as a classic is under question