Well, the one that most people would probably think of immediately is in Highpool. If you mucked around in the little stream, you'd eventually slip on some rocks and fall, causing kids to laugh at you. If you wanted, you could murder those children and, if you kept going, eventually kill all the kids at the camp, and then kill Red Ryder who showed up to avenge them. Doing so would turn the Highpool from a lush canyon to a completely desolate dustbowl. The stream and pond dry up and vanish, the dormitories become dilapidated husks, the doctor (the cheapest in the game) vanishes, the trees die, etc. It's actually pretty horrifying when it happens, and nothing you can do (aside from reloading) can ever bring Highpool back; you've made the world more desolate because you're a jerk.
There are also plenty of smaller things throughout the game. Now, this was in '88, so it was somewhat limited in global effects, but the choices you made did have an effect on the path you took. You could accept Ugly's offer and let him get away, or you could gun him down and run the risk of the Mayor's wife being blown to pieces. You could rescue Jackie from the cave, but you had to kill Bobby's dog to do so, and if you did, he'd be after your blood. Things like that.
Alwa nasci korliri das.