Same here, (aside from superficial reasons) I really liked her in Iron Man 2. In this movie I thought her best scenes were the interrogation scene, her scene with Loki, scene with Hawkeye and her scene in "recruiting" Banner. I guess, really, all of her key scenes seemed very well done and played by her.
Her action scene "It's gonna be fun" to get to the key moment of the movie where she closes the portal. She was integral at every step.
Trick the Master Trickster into showing his hand
Get Hawkeye back under control
Close the Portal
Black Widow was not just idle eye candy in this movie at all.