You can learn a lot, watching dogs play; one thing you can learn is to appreciate how important body language is to communication. Dogs, unlike people, aren't burdened by the spoken language. They communicate with posturing, subtle shifts in the fur, or not-so subtle changes in stance. My dog can emit a very deep bark when she puts her mind to it, but when she does it from a "play bow", the meaning of that bark is quite different from when she's doing it standing bolt upright. The teeth she shows when wrestling are the same ones she's shown when defending herself. It's the posture that makes everything clear.
Some owners fail to understand this, which can create two problems. One is that they will assume that there is a problem when there isn't one. They watch two dogs wrestling, or see one dog barking at another, and assume that it's all bad. They've transferred our societal aversion to violence over to the canine world, and assume that because they don't think kids should play rough that dogs shouldn't either.
The flip side is that some owners don't know when things are getting out of hand. They'll dismiss concerns about a dog's behavior by saying "they're just playing" or "they'll sort it out...they're fine!", when those things aren't true. If you understand canine body language even vaguely, you can recognize when things, are in fact, not fine. When tension is mounting, when one dog is starting to move from playing into "get the fuck away from me" mode, and when conflict seems to be getting inevitable.
If you can recognize this, it creates an interesting question for the owner; when do you decide to intervene in the conflict? Step in too early, and you're robbing your dog of some play time, and possibility teaching it the wrong thing about canine social behavior. Wait too long, and you have to break up a dog fight, which sucks for everyone. There seems to be an art to recognizing when things are calming down, or when things are about to go overboard and you need to step in.
Obviously, I think there's a great deal of carryover to these ideas for dealing with human conflict. Of course, there aren't "human parks", but there are plenty of opportunities in daily life to observe people's body language. Watching them get confrontational regularly probably requires working in a bar, or a prison. Maybe a high school (though I imagine that kids might have some different posturing that goes on. But maybe not). Still, I think the basic idea carries. Tony Blauer emphasizes the importance of understanding body language in the PDR Program. I remember hearing an interview with an author who mentioned that there was some kind of online database of human body language, but either it doesn't exist any more, or my Google fu is weak today. It's worth looking into, at any rate.