Sally Struthers's, uh, use of her hands
I was amazed to see a forty-year-old film be this explicit (or very close to explicit) with something you don't see presented much even in modern day films (non-porn). First, she demonstrated her technique on the gun barrel; then she gave a very hot performance where it was pretty obvious what she was doing in the back seat, the coat over Rudy's lap (and her husband in the front seat--ouch).
Or maybe it wasn't as obvious to everyone and that's why I see so little comment on it? Google turns up only one mention that I can find, and none of the thread subject lines reference it here.