The night sky and stars look convincing. It was a full moon at the start of my game, so it was easy to see. I don't know if that will change.
The ambient noises and animals are fantastic. They animate impressively and the sound effects from the horses to the dogs to the chickens, are spot on, very realistic sounding. I wandered into a creek area at night and you don't feel alone, everything feels alive. The first area appears to be blockaded. In typical GTA fashion, all of the bridges are out.
I got to one bridge on the eastern side of the first territory, and OMG the water down below in this river looks incredible. The shaders reflecting the sunlight and the ripples in the water looked amazing.
I've noticed some occasional pop in, but it's minor compared to GTA4. Galloping around on a horse, the grass doesn't pop up in front of you, it loads it all smoothly, only once in a while you'll notice something on the horizon pop up or as you pan the camera quickly.