Question about dog park etiquette... sorry so long?

I think you did the right thing. The person with the aggressive dog was right in immediately removing himself and his dogs from the situation. You don't want your dog to think that you go to the park, he gets attacked and then you go home. You showed him that it's supposed to be fun. I think these things happen frequently at dog parks, and it's really important that people should know that this is no way to introduce strange dogs to each other. The average person has a really hard time reading canine body language. Perked up ears, a towering stance, intense staring and an erect tail are all signs of impending aggression, but people don't notice it. And nobody thinks their dog will hurt another until it happens. It's a risk that you take everytime you go to a dog park. I know that purpose of these places is to allow the dogs to run and play, but the lack of ability to control the dogs is a safety issue. My dog was once aggressive in a dog park and since she didn't have a leash on, I couldn't catch her or stop her. She didn't attack the dog, but chased her until she ran into the fence and hurt her leg. We never returned, and never will. I would really take a look at who's in the park when you get there, and then make the decision on whether you go in or not. If anyone looks a little too interested, politely ask the owner to put the dog on a leash for a controlled introduction.