As I understand it, flying in a park is fine as long as the part of the park you're flying in doesn't have people wandering around in it. Flying over people is not ok under the AMA safety code. So that might rule out some parks, but allow others.

I've flown small electrics in backyards, parks, construction areas (when they weren't working), abandoned lots, and school yards. But only there weren't people in the section I was flying over. Heck, if nothing else having someone walk on to your "runway" when you're low on battery/fuel and on final is a good way to wreck the plane even if you don't hurt someone.

Flying from roads and parking lots is always interesting, but not necessarily unsafe, it depends entirely on the situation and who is around.

One of the things about parks is that model aircraft tend to be kid magnets, and kids will do things like run out in front of a landing plane, which can make populated parks tricky. It helps if you fly with a friend or three, and have at least one person spotting and doing crowd control so the guy flying can concentrate on the plane.