The dog park can be a great place for some dogs. One of my concerns is the lack of interaction I often witness, between dog and owner. Too many people bring ill mannered dogs to the park and allow them to interact uninterrupted, with other dogs . 
Not all dogs are dog park material. Know your dog, know the dogs who frequent the park you go to. Just as with people, not all dogs get along; some are bullies, some are softer than others, some simply do not want to interact with other dogs.
Here is a great video illustrating some dog park examples of poor behaviour and lack of much needed intervention.