Music, sounds, visuals, how the animals move, etc. (with the limitations of AI obviously) all contribute to creating a fantastic environment. If you burn an animal while hunting it, it will slide and set fire to the ground it slid onto... not that I like to burn animals alive, but if they come attack me while I am minding my business then I have no choice but to defend myself. But then you realise, wait a minute, that tiger was scared of fire, is this game going to be realistic and let me scare other animals away? And it does, you just point your fiery club towards them.
You get to tame animals, craft everything you own, build your village - I am guessing these are the elements that players of the previous Far Cry games feel are more of the same. As I said, being the first Far Cry game for me, I find this all very rewarding.
All in all, it's a beautiful, immersive experience, I highly recommend this game.