Always something of a fine line on the crowd bit. It's like porn, you know it when you see it. I don't think anyone expects complete reverent silence all the time, the spontaneity of the crowd can be half the fun.
It's one thing to make a comment to your friend during a song and another to carry on a complete and unrelated conversation as if that were the radio you were listening to. I think the rudeness comes from people wanting to have it both ways. If you want to talk with a friend or are just there for some background music, that's cool, go to the back or downstairs. If you want to enjoy the music, get in the crowd and enjoy the music. Just don't spend half the song screaming to your friend, "this is such an awesome song!!!"