Light
Here's another entry in the vein of random opinions in science : of all the forms of pollution out there, light pollution probably seems the least pressing. When children's toys contain coma-inducing drugs, when Lake Tahoe is losing it's splendor and clarity, when all cities of the world are enshrouded in brown mist (which I remember all too well looking back on Boston while taking a harbor cruise one day), why worry about blazing street-lamps?
I think it's something primal. Imagine just a couple hundred years ago (or a few thousand or million for that matter): People looked up at a sky that blazed with the undiluted brilliance of bazillions of stars. The brightness, majesty and sheer number of stars they observed is really beyond any appropriate words and it's only something we can imagine today.
It's no wonder that recent cultures placed so much value and religious conviction in the stars. The night sky was the most spectacular thing human eyes could witness not just for recent generations, but for millions of years of evolution.
I've gotten attached to this whacky idea that we have a primal need to enjoy that light show in its full non-washed-out glory, that this need is somehow hardwired into our DNA. But today, even in the most favorable cases, we are unable to see the night sky that our ancestors saw. I guess Pixar will render it one day in a movie - and I suppose we could go to planetariums to see something like it - but the problem is that we have no idea what it's supposed to look like so anything will just be a guess.
It's better to recycle your plastics, glass and metals. It's better to buy a hybrid car, carpool, or walk/bike to work. It's better to buy pesticide/steroid free groceries at every turn. It's better to install a solar panel. But if you are so compelled, turn off the lights too.