We don't let our two (DS is 5, DD is almost 2.5) play outdoors unsupervised...primarily because
- our neighbors have rather large dogs running loose and
- we live on a bend in the road, and you can't see what's there until you're on top of it. (Well, that and people on the road drive as if it's a race or something.)
I practice the normal parenting safety: hold hands and look both ways crossing the street, don't run on the sidewalk by the road (my two are clumsy, what can I say), don't go into the road (see: crazy drivers), and for the love of Pete, don't try to reach for the pots when I'm cooking!
They're at liberty to tromp up and down the stairs to play, holding the rail or not. We don't panic when it starts raining (I'm the one who goes out to watch the lightning, so...wait, bad example), in fact, I encourage them to get wet. They'll dry! We get good and filthy outside, look at bugs, play in the dirt, and generally have a grand time.
(Mostly, I worry about spiders. But that's only because I'm terrified of them. On webs, away from me, no. Otherwise, ye gods!)
There's an advert here in the States where the mother wraps her children in bubble wrap before they leave the house. Sometimes, I think people would do that if it wouldn't look completely stupid.