I take my Vizsla to the Paw Park once or twice a week, and she loves it. I have seen a few instances of aggression from other dogs but they are usually very rare. Like another poster said, you just have to let the dogs figure out the dominance pecking order and don't interfere. Don't freak out if another dog start barking at your dog or steals its ball. There can be some growling, ear-pulling, etc. that never amounts to injury. The only time I've seen an all out dog fight was when an owner had their dog on a leash, and when dogs feel restricted on the leash they often lash out. Some times it happens and its annoying, but if an overly horny male is not controlled its owner and starts trying to hump your dog and pin it down, its acceptable for you to grab that male and throw him off

The dogs that seem to have the most problems initially are the ones that are extremely submissive, my dog was one of these and got "picked on" quite a bit, but she gradually gained confidence and after taking her there 8 - 10 times she has become one of the most social dogs out there and absolutely LOVES going out there.
Taking your dog to the park and seeing them play with other dogs and run with joy and abandon is one of the funnest things about owning a dog, I think if you don't take your dog to the park out of fear you are really missing out.