The Black Widow is one of the most bad ass Avengers. Can she turn into a giant green monster capable of destroying a building with her bare hands? No. Does she have super serum genes that give her incredible strength? No. Does she have a robotic suit that somehow gives her god-like superpowers like Iron Man? No. She has no superpowers, and no special armor, yet she still fights all the supervillains just like everyone else. You might argue that Hawkeye is similar, but he at least has a bow and arrow.
She's also cunning. Look at the first Avengers movie. She even fools Loki, the god of mischief, into telling her his plan. She's so cunning that she might ask you, "what do you want to eat?" and you can't be sure if she's laying some trap for you.