Making the driving more realistic, and thus less fun. Making insane stunts and persuits was one of the funniest things in the III series. In IV? Not so much.
Same with gun firing and random killing.
So, less fun driving and less fun killing. Don't know about you, but that kind of describes GTA to me. Hence the lack of fun in GTA IV.
I don't know about you, but fun combined with realism is not a contradiction.
Sure, if you're stuck in a wasteland and think you have to find water and drink every 10 min lest your character gets dead, then that kind of realism is not fun.
But having really cool driving physics that just feels "right" (i.e. realistic), dealing with characters that you can have empathy with (i.e. not the "fun" bat**** insane moron type a'la Saints Row), then I endorse this kind of movement to more realism.
But whatever mang, whatever.