I've been to a lot of dog parks and read their rules. Over all, I think pretty much anything is ok, as long as you are not creating a problem for yourself, an animal, or another person.

The best rules I have come across which seem to be universal around here and just plan common sense.

1.) Must be vaccinated, and of course, up to date.
2.) Really should be spayed/neutered. Unneutered males can sometimes cause real problems, specially unaltered females around various males whom you don't know may or may not be neutered regaurdless if she is in heat or not. However, it's ok to bring them if you know you can control your dog, has proven that he/she will not have over whemling territorial/dominance issues, and there are always lots of times you can go to the park and you are the only one there.
3.) Must have collar/harness on at all times, and you have the lead on you at all times.
4.) Your dog really should be at least decently reliably trained lol.
5.) SCOOP YOUR POOP
6.) If your dog shows aggression remove immediatly.
7.) If your dog is overly fearfull while at the park, remove your dog and try a different approach to dog park introductions, an overly submissive/frightened dog can cause as many problems as an aggressive one.
8.) You really don't need to be there if you don't have a dog.
9.) Pay attention to your dog, and dogs around your dog.
10.) No addmission under 16, and then some of your 50 and irresponcible.
11.) Watch out for everyone else and their animals, it's your best defense for your own safety. Most people do not know the dog park rules.