I've been in a few moshes now, the craziest was a Black Dahlia Murder one that got out of hand and I got out of there. It ruins the spirit of the show when people actually start getting angry and punching each other, and you see a big fat guy bulldoze like 5 people at once and in the process hurting like 10 people. Based on a true story.
I myself like to headbang and occasionally go a little crazy (as you'd expect from a metal show), but when I can't actually listen to the music and watch the show because assholes are either moshing like fucking idiots or emo kids are practicing their fucking kung fu dancing, I get annoyed.