I've been revisiting the game recently and I honestly can't think of a more impressive open world in gaming. It's vast, beautiful and absolutely chock full of life.
The attention to detail is absolutely staggering and, for me, nothing emphasizes that more than the way the animals live and act within the World.
I know wildlife in open world games isn't exactly a new thing, but I can't remember seeing another game in which the wildlife exists and interacts in such a lifelike way. I'm not particularly technical but I can't imagine the effort involved in animating and scripting all those different species.
Just as an example, look at the alligators. Not content on having just one model, you have Alligators of different sizes and colourations. Some are in the water, some are on the banks. Some are guarding nests. Some will attack if you get near, some will scuttle off into the water - the diversity is mindblowing.
As I play at a more relaxed pace to the first time around, I'm noticing new things all the time. Just this week I realised if you shoot into a bush all the insects will fly out, and then I saw a deer trying to rouse it's dead mate. .
Most discussions on the game I've seen seem to focus on the rather polarising gameplay, controls and pace elements, and whilst I agree there are a few issues, the actual World seems to be almost taken for granted.