I still have and occasionally play COD3 on my PS3. I like the online better than MW/2/Haven't played BO.
The maps in COD3 are much larger (Or just have more open space). The past few CODs just feel kinda crowded in the MP maps to me.
It's hard to move around without being seen or shot down, and it seems you have to be very stealthy in the past few games if you want to survive. It doesn't bother me much. But I'm not good at it most of the time :P
I still like the newer COD games, they are a lot of fun. I just happen to like COD3 more than them at this point, and I have yet to try Black Ops.
And nobody has firefights in the actual open areas which are no man's land anyway.
But I can see why, due to area of where we are fighting in now/in the past few COD games (Setting, area of world, technology since older games).
Feel free to prove me wrong/share your opinion on any of this, it's just my opinion.