Hmmm so many to list here.
1.) Puddles created by rain at corners or buildings or the edges of the streets
2.) Can put dents in a sports cars frame by kicking it
3.) peoples hair gets wet in the rain and they react realistically to the fact that it is raining. Covering their heads with whatever they may have, jogging for cover, etc.
4.) The realistic looking blood splatters on your clothing when shot
5.) Cuts and scars sticking around for a good while on your character.
6.) If you slam front first in your car enough times, it will begin to have trouble starting or hitting max speeds. Also, tearing up your front / back axels will stop the wheel from spinning, eventually burning them out or blowing them
7.) The fuel leak from a busted gas tank, when lit on fire, the trail of fuel will also ignite
8.) if you walk into water with say, only thigh high, only that portion of your clothes will get wet. Same with ankle high water, etc.
9.) If you land in the water with your car and stay inside, it will eventually flood and start to sink and go underwater.
Just hundreds of little details like that I could mention