The coolest outdoor place I have seen would have to be in the longest road. It's a hole in the road with a blown up convoy truck. Even the trees near it are bent from what appears to be an explosion.
I also like that omni outpost north of 20k simply for the fact that it plays a sound of radio com chatter when you enter it. It's really not that impressive to most people, but the reason I list it here is because it is the single most imersive thing I have ever come across in AO. It really felt as if I had entered a guard outpost because of that radio com.
As you can see I favor the little touches that bring the world to life more than the grand things... I am interested to hear what everyone else considers their favorite outdoor sights.