Did you see the counterpoint? While the guy that wrote it is quite rude (inappropriate to call children "stupid") it has a lot of merit.
Since my babe is only 2ish the whole leaving your kid alone thing doesn't come into play yet, but living in NYC, surrounded by lots and lots and lots and lots of people I have already had to deal with some of this type of stuff in playgrounds.
1) People don't shut playground gates. Why? Maybe their kids are old enough to understand how to stay in the playground and not, you know, run out into the street. Maybe their kids stick around them rather than make a beeline for the perimeter and all gates.
2) Playgrounds in NYC are often for multiple age groups. And let me tell you, some of these kids (6-10) (who's parents are waaaayy over there hanging out drinking coffee and chatting) are telling me that the giant (GIANT) jungle gym is where they are playing and that my kid is too little and he can't be there. Funny enough, this has happened enough times for me to believe that the self-entitled parents are passing that self-entitlement onto their kids.
3) Older children on swings are not looking out for toddlers. Period.
4) My kid bites. I'm working on it, daycare is working on it. It's just him and it will pass but I have to keep a close eye on him at the playground. Many other parents have said to me that they believe in they "just let them work it out" philosophy. I know for a fact that they would not be quite as happy when my kid bites theirs. I thought I believed in that philosophy, until I had a kid that bites. I continue to try to let him figure stuff out on his own but it's difficult.
My favorite playground in Prospect Park Brooklyn is called The Imagination Playground http://www.prospectpark.org/visit/places/play#imagination
Despite it being rated #1 somewhere (Time Out NY?) it's often not very crowded.
Thing is - this is a playground where you can let your kid roam. There are only 2 entrances and they are more often than not kept closed. There are a few things to play on with, a stage, but no slides etc. I've always had a much better interactions with families in this playground than the other playgrounds. I've seen kids from 12 mo to 12 years play together and/or simultaneously. And the older kids are usually looking out for the younger ones - whether or not they're siblings - not ot telling them that they can't play on something or another.
So to summarize this ramble. I love the idea of the "leave your kids in the park day" but in NYC at least I just don't think it's a good idea.