IMO the ScarJo pick squanders the potential of the character. The way she walks and talks lacks the edge and the style that could make Black Widow a total badass. She lacks that 'femme fatale' quality that Romjin had for Mystique (not Lawrence) or Traue had for Faora.
ScarJo just doesn't seem cold enough to me. I don't believe it for a second when she says "love is for children". Using Agents of SHIELD terms, she's too much Skye, not enough Ming Na Wen, if you know what I mean.