I'm incrediblely late, but I think Scarlett Johanssen was terrific as Black Widow. It was basically her introduction to the Marvel Film Universe and she made me excited to see her in future films.
Her fight scene in the hallway was the 2nd best fight scene in the movie, the first was the Hammer Drones battle in my opionion. Anyways, everything from her walk to her emotions screamed "sexy but deadly".
Scarlett's butt looked very nice in that tight fitting outfit as well