Ms. Ehle did the best she could with the script she was given—indeed probably put more into Betsy than was on the page—because they reduced her character to a impulsive, indecisive, shallow woman. The book version of Betsy was certainly indecisive and impulsive, but not shallow. She struggled against real concerns, motivated by real insecurities and past history, all of which were lost in the movie. And the character of Abel, although more substantive, lost all of the juicy grayness that made him so interesting.
The movie was beautifully filmed, absolutely gorgeous, and it did manage some of the wonder of the book. They stayed true to its spirit—I have to give them that. That isn't always the case in film adaptations. They didn't ruin the book.
And here's the BroadwayWorld forum thread about the announcement of Ms Ehle as Lady Macbeth. It's mainly lots of "yays" so far, but worth keeping an eye on.