2. City needs to remember
In GTA 4 we've all run someone over and driven off without a care in the world or shot someone in the street and not blinked an eye-lid. What if the people remembered? That woman you hit with your car might have had some gangster brothers, who have now made it their life's goal to hunt you down. That building you blew up caused a whole street to be blocked with rubble, and it takes the clean up crew a whole week to sort it out - the rubble doesn't just disappear. The closed road resulted in a massive traffic jam on the adjoining road. Little things like this will make the game world far more believable.