The only audience reactions that I can vividly recall were a few muffled sniffles when Spock bought it. Even my grandmother got misty eyed at that point and I don't think she even knew too much about the show; It was just that powerful. I on the other hand didn't, I was only twelve at the time and was more disturbed by the Ceti eels. I also remember telling her on the drive home not to be sad and that Spock would be back since he did something to Bones before he died. I did however make up for my callousness that same year when I cried like a little wussy man when E.T. died, as was the custom back in the day. As to "Khaaaaan", it's probably just such a great and iconic line that it's simply taken on a life of it's own over the years.