So my local dog park is actually quite nice with quite a few great dog owners that frequent it. It has a dedicated park for large dogs (20lbs+) and a small dog park.
I usually go on the weekends, and I have never seen any problematic dogs there. The owners know their dogs, and are actually pretty good.
My problem is the people that bring their 10lb chihuahuas into the large dog area. It sucks because when I see a small dog in there, it means my greyhound cannot go in and play.
She has some prey drive in her, and combined with years of racing (chasing small fast moving objects), I do not want to take a chance that she will snap back into a racing mindset. Any larger dog, my dog does perfectly fine with.
So why do people do this? The small dog park is smaller, but still a nice size. 9 times out of 10, if I watch those people and their dogs, they won't leave their owners side as the 5-10 large dogs running around intimidate them. Then I look at the small dog side, and the 3-4 little dogs are all running around having fun.