Witherspoon miscast
She just kind of makes Becky sincere in every scene. The movie ends up being rather unshaped, and the energy just wheezes out of it.
Becky is a schemer. Miriam Hopkins had more fun with the role in 1935. And I guess you could count Vivien Leigh as Scarlett O'Hara, since Margaret Mitchell had one eye on Vanit Fair as she wrote GWTW.
Related Article:
http://www.rottentomatoes.com/m/vanity_fair/articles/1324659/i_knew_sc arlett_ohara_and_you_ms_witherspoon_are_no_scarlett_ohara