I just played GTA, its totally unrealistic. I hit a guy in a car and he flipped so high he landed on a 3 story building's roof.
And in MW2 they're screaming uncontrollably and the terrorists shoot the ones crawling and the ones who surrendered. Its different...
Dude, GTA IV's pedestrian AI is that of a real life reaction, at any moment you can get out of your car and bust some caps in people, and people around them will panic and fear, run, or get out their cell phones and call police. And the police respond, Even if you're in a building like that hospital or lawyers office, or even in the street the police/SWAT observe their surroundings and keep their heads down and move with tactics, and when they see their buddies getting killed they give commands.
In that single mission of modern warfare 2, it's just a big trigger. MW2 is not as opened as GTA.