Usually it's beer dregs that sail over my head at gigs......why people throw stuff I don't know. I remember seeing Placebo at V fest years ago, and Brian the lead singer having a massive swear at the people throwing stuff. There's just no need for it, people should be there for the sheer love of music, not for the purpose of annoying other fans.
The punk gigs I've been to are among the most well-mannered in terms of the crowd. It's usually pop and mainstream rock that you get the morons.