Dog parks can be a dog owning city dweller's solution to a space challenged environment so they can provide for their canines exercise needs. Urban budgets usually do not allow for dog park supervisory staff. There is no one in most parks checking on the doggie guests coming in, whether people are cleaning up after themselves, or available to safely and knowledgeably break up an inevitable fight. Vaccination or pest control medical records are not checked before people come in.
The population of individual dogs and personalities changes all the time in these parks. Not all dogs are going to get along with any strange dog that walks up to them, even if they get along with most dogs. Some dogs are reactionary around toys and strange dogs, some dogs just don't like certain other dog personalities, and some dogs are jealous of attention taken away from their owner or live in canine companion. Combine with this the changing pack order that happens, and dog wanting to naturally maintain or protect their status. The changing of the pack alone can cause problems of aggression and fights. This might not be a problem with your dog, but remember you are surrounded by other strange dogs that this might be a problem with. No one wants their dog to get in the mix to simply defend themselves.
Most dogs that go to dog parks are not trained. Here I am using my definition of trained, not the mere assuming the position of sit for a second and then releasing. Training is what makes a dog more likely to be controllable when unpredictable situations occur. Training allows most of us to keep our dogs safe, and it's why most trained dogs don't need the confines of a dog park to get their exercise. Untrained dogs have no one to follow but themselves when a problem arises. The communication between the owner and dog for direction is non existent. This will only add to the problem above if a dog to dog or person to dog problem breaks out.
Finally, as intimated above, disease and pests come into the dog park. Even waste that is picked up can leave behind germs, which can be a problem if you don't know what the many dogs that come through there might have. Because of the lack of supervision, sometimes these parks are nasty and dirty. Fleas are a pest that is common to be picked up at a dog park. An owner will never know if a dog has been fully vaccinated or potentially exposed to something like rabies (if there is a supervised dog park, this issue might be solved).
The best solution is to train your dog so that you don't need a "dog park" to visit. Also, have friends whose dogs you know get together with you and your dog. Be sure that your dog is in control through training, when you walk them around strange dogs. This is the kind of socializing dogs need, not instances that increase danger to your dog. I would love to hear about supervised dog parks, please e-mail me or comment if you know of any.