Parents who hover over your kids at the playground and panic over small tumbles & messes: please stop. If you want to impair your own kid's social, physical, and mental development, that's your business, but when you do it in public you're also messing up *my* kid.
When I'm responsible for a young kid, I do step in to stop them from hitting others. Aggression is wrong & toddlers don't know that without being taught. Accidental jostles and play-fighting don't need parental involvement though, IMO.