I was very fortunate to have access to a good ( but informal ) dog park for my lab's first 3 years. It was a great tool for socializing and the place had it's own unwritten code of conduct that for the most part worked. Very seldom did I see aggressive dogs and the times I did, their owners were brought up short by a mass of disapproval.
The ones with the very small dogs watched that they were not run over, but other than that , it was a free-for-all. That may sound ridiculous, but to them it was pure heaven and it was obvious. It might depend on the set-up also. This 'park' was maybe a 30 acre hillside bordered by a stream within hundreds of acres of city park, so it had room for various comfort zones.
It was neat watching doggy dynamics. You can learn a lot about your dog in that situation that you would rarely be able to any other way. Be responsible though. Don't let your dog be the one to ruin a good thing. It won't mean he's bad or anything, just means that they aren't for everyone.