I don't go to dog parks because Maple doesn't deal well with other dogs she doesn't know, especially ones that won't leave her alone. Woody couldn't care less, and would only run the fence line chasing birds flying by.
I agree that if you can not control your dog, you should not have it at a dog park, especially if it is agressive at all.
I do walk my dogs daily through several parks in my town, and am amazed at all the idiots who let their dogs off leash and as the dog is running over to me just say, "It's OK, he's friendly". Um, my dog is not (well, she is, but, she feels the need to protect me and Woody from all dogs), and, when she's on a leash we shouldn't have to deal with loose dogs (there is a leash law here & we have multiple dog parks nearby). I pay attention to what's going on around me and either change direction to avoid other dogs or use the leave it command when another dog approaches and it works well.
I have animal control on my speed dial now because there is one dog in particular that charges us barking and growling and the man just doesn't get it that this is unacceptable (Ugh, and I never know which park he'll be at in order to avoid it).
I do my best to be a responsible dog owner, and to protect my dog too.
I think dog parks are great for those of you who have well socialized dogs and wish I had done a better job with Maple.
Sorry, kinda got bloggy.