I go to the dog park a lot with my dog and have noticed that GSD's don't play the same way that other dogs do. They are "mouthy", "nip" at each other (very oral players...is the best way I can describe it). Some people don't take well to that. They think they are being agressive. With Jericho, he will sometimes pick out a dog that is kind of timmid and just be a total jerk to the poor thing. For that he get's bitter apple and a "break" from playing. Usually, that is enough to divert his focus away from that dog and back on the one's who enjoy playing with him. I wouldn't muzzle your dog at the park, but maybe if the park is really busy be ready to leave early. I would still try and see if it's going to be a good play day, but be prepared to leave if you have to. I also go to the same dog park every time, just so the crowd is more predictable and will be familiar with how your pup plays. I think it's really important, for Jericho at least, to go and play with the other dogs so he learns how to play respectfully with other dogs.