The characters will walk differently based on where they are. If they're on soft mud their feet sink in and they trudge through it.
The engine noises are SO much better than in GTA4. Big V8s have an appropriately throaty exhaust note, exotic V12s SCREAM and you can hear the blow-off valves on turbo cars. High-horsepower cars spit fire out the exhaust and I've even heard that oh so wonderful burbling noise when you let off the gas after running hard, those little pops and crackles from the overrun. And the car's exhaust tone will change if you put different exhaust systems on it.
A bird strike in a plane is a BAD THING.
The fact that some cars now have working convertible tops still blows my mind.
Elk will freeze if you catch them in your headlights at night.