You have to understand this is an opinion. It's because for me it's not a worthy succesor to the series. For me it lowered the bar for what I want an FPS to be:
1 - Immersion. Where COD1 and 2 made you think you were in the middle of a war, COD4 failed.
2 - Where COD1 and 2 weapons were so cool and amazing to shoot, COD4 ones were easy and overpowered making COD4 campaing just a point and click game.
3 - COD4 was a console game for the most part, which was obvious by what I said in 2 and how the enemies reacted, easy to kill.
4 - Half the gaming time is spent in stupid gimmick missions, scripted to make things even worse, like the one on the gunner plane and the sniper one. Not that the one on the boat is better.
5 - Plot? What plot? Oh yeah, patriotism is so cool it's a plot by itself...
As I said it's my opinion. I really hope never ever anyone tries to make their game following COD4's standards.