jiminy, good quote.

I think that different breeds have different temperments, just like diffent people do. Some are more prone to act agressively if threatened and some are more prone to back off and be submissive. Whether they act on their instincts though is largely a result of training. Pit Bulls are still widely trained to fight, and to fight to kill. This is an unfortunate effect of humans' selfishness. It still goes on a lot, and it's sick. But it does make a large number of Pit Bulls very dangerous as a result.
As far as not knowing whether a dog is mean or docile, I think it's safe to say that no one should ever go up to any dog without first asking the owner (and no owner should have their dog loose.) Even a normally-gentle Standard Poodle could bite if it had been trained badly or was frightened. And all parents should make sure their children know this. I can't tell you how many times little kids have run up to my dog and their parents weren't even watching them. Fortunately for me, I can totally trust my dog to be extremely gentle with kids. I've had kids pull her hair and bug her and the worst she'd do would be to whine and try to get away or hide behind me. But not all dogs are like that, and I'd never take a chance.
~ashes