Black Widow doesn't remind me of Buffy so much as Zoe from Firefly. Both are terse, tough and completely loyal soldier-types who will take zero guff from anyone. Also: when you see how scared Zoe is of the Reavers, it brings home how truly terrifying they are. Ditto Hulk; seeing Black Widow (who so casually fought three men while tired to a chair) authentically scared of Bruce Banner in India underlines how truly scary he is, even in his rumpled professor guise.
Loki vs. Hulk has echoes of Buffy taking on Glory. When Buffy in season five is fighting Glory, going to town on her with this awesome hammer, Glory's all: "you can't defeat me! I am a god!" and Buffy responds by beating her down. Heh, puny god!
Loki trying to mind-whammy Stark and failing: same thing happens in Serenity. We see an early scene of the Operative paralyzing a victim by pinching a nerve in his back. He tries the same move on Mal in their end fight, only it doesn't work because Mal had suffered a war injury that killed all sensation in that area... "This usually works."
And the Black Widow interrogation scene with Loki: pretty much every scene in the Whedonverse where a powerful male figure underestimates a woman, assuming she's weak, and she neatly flips the power dynamic around.