It's exactly like you said, "a strange mix between violence and friendliness". I'm usually friends with nearly every single person in the room but much of what happens would constitute assault in any other context. People still pick each other up when they fall, even that person just punched them in the back of the head. I suspect you'd find very little has actually changed.

On a semi-tangential note: It's always so amusing trying to explain this sort of thing to someone who has never been a part of punk. Another commenter in this thread said that it's self regulating. And that's what so many people don't get. There are rules. Unspoken ones, but they are there. Everyone in the room knows when something is actually out of hand. It may look like total mayhem at times to the untrained eye but if you've been around long enough you see the same sort of organic patterns of behavior at every show. Sometimes I'm inclined to think you've never really connected with others until you've been to a punk show with a hundred other like minded people crammed into a tiny dim smelly space with terrible music playing as loud as the speakers will allow, all the while loving every moment.