Love all the little touches in the game, just a few I've noticed and/or read about:
Pressing right on the d-pad you can talk to people, and keep pressing to have a conversation with some. Some of the dialogue is quite funny.
The prius-like car (dilettante) switches from battery to engine power just like the real one, you can hear it.
If you shoot out someone's tyre, a lot of the time they will pull over and change it for a spare.
If you keep revving next to a sports car at traffic lights they'll race you (they even jump the red light sometimes).
I was driving a Veryon-like car, when I pulled up somebody walking past stopped and took a photo of it on their phone.
If you walk around carrying a gun, some people will call the police.
In a tunnel, car engine/exhaust noise reverberates realistically and GPS signal is lost.