I just read a few articles over at IGN on their multiplayer take of GTA IV, and damn I can't wait. In every game mode you have all of Liberty City to run free in, no invisible walls, no doors that won't open, just free reign.
There is even a mode called "Free Mode" that alows for you and up to 15 of your closest friends to do whatever the hell you wanna do. Think of this mode as single-player minus the story, so you still have all the fun stuff, cops, pedestrians, guns, cars, helicopters, motorcycles, etc...
So what can you do in Free Mode, kill each other? Sure you could, but think about it as co-op in a way.
My favorite thing to do in a GTA game is all the ancillary stuff, jacking cars, driving around, shooting random people, running people over, getting a wanted level and running from the cops. Now wouldn't that be so much more fun if I could do that with a buddy? How about 4 buddies or 15?
If you are interesting in GTA's multiplayer at all I would highly recommend going over to IGN and giving part 1 and 2 of their multiplayer coverage a read.