I don't think Scarlett is a terrible actress. She's been choosing some awful 'mainstream' pictures lately. She needs to go back to independent films with well-written roles for her. It's a wasted talent.
agree with you, scarlett did very bad choices in the last three/four years, acting-wise... however she was a fantastic actress when she was still starting her career as an adult actress, examples are ghost world, the girl with the pearl earring and lost in traslation... then, she got lost in the hollywood mainstream focusing herself on her external appareance and her role as a sex symbol rather than on her acting.
As well as Natalie Portman, she does a bad acting when it comes to a movie with an unsustainable script. I think they are mainly script actress, they follow every step the director/filmmaker/scripter has to say; they work with patterns.. instead of adding some personal touch and some improvisation... in the case of Scarlett I don't know, but Woody Allen when he worked with Portman had some problems with her (she was still a teenager) in these terms: she wasn't able to offer her personal impression/view of the character and the film (Allen works a lot with actors improvisation, not only on the script but also on the way they move).
btw, I read not so long ago, that Scarlett will be playing as Marilyn on a movie... now I just wonder if she was an option for the Simon Curtis' movie or there's going to be two movies about Marilyn soon and simultaneously (as it happens in Hollywood very often... for instance, with Truman Capote in 2006).
I think Michelle is a great choice.