There's also a real difference in gameplay design. The old GTA games used to give you vague mission outlines and leave the particulars up to you. They'd tell you to kill a guy, and you could hole up with a sniper rifle, dash in guns blazing, run him over with a car, drive-by him into oblivion, park a car with a bomb in it in his way, etc. The results were usually hilarious because of the way the open world reacted to what you did in it.
GTA IV's missions are almost all heavily-scripted corridor shootouts. There was rarely any room for you to stray from the mission designer's plan. Even car chases had scripted endpoints - enemy cars would be invincible, and they'd magically speed up if you got too close to them, until you'd seen all the reversing garbage trucks and collapsing log piles the designers wanted you to see, after which point you'd be allowed to finish the mission.
Unfortunately, GTA IV was so influential that that's just how all open-world games are made now. Just Cause 2 is one of the few games this generation that gives me that oldschool GTA feeling of freedom. I hope they can get back to the old goodness in GTA V, even if it's just a few missions.