They're fun and intense and do occasionally result in minor injury. But (usually) they're extremely good natured, with people nearby immediately stopping to assist anyone who looses their footing or wants to get out. They are also typically ringed by "enforcers" who participate by standing their ground and helping block people on the sidelines who would obviously rather not be bodyslammed.
There's always a few douchebags who want to throw punches (the "crowd killers" the article speaks of) but usually they get moderated out pretty quickly. Fundamentally a pit is not about hurting people, it's about responding to the music in a extremely kinetic way. Yes, you're slamming into other people, but you're not trying to hurt them, you're generating a shared sense of physical intensity to match the music. It's dancing. Really rough dancing.
If you have the kind of physical pain that the author regularly does, I don't really know what to tell you. Don't stand in the middle front? The pit never takes up the entire floor and if you maintain a little awareness it's easy to stay out of. And if you look scared and are obviously trying to get out you'll immediately be surrounded by several big friendly dudes opening a path for you.
If you want to find this without the descent into the Dionysian, go climbing. The world focuses down to a single, narrow point: "find the next hold".