Lag, match making specifically join in progress, party system, unbalanced perks, terrible map design, terrible weapon balance thanks to horrible map design, terrible teammates, and overall swapping to MLG mentality.
Definitely spawns. Everything could just be fixed if there were less players per lobby and if they added "difference in height" to the spawn formula. I have been killed by so many people who have spawned right behind me on a lower height and I have been spawned the same way too many times. This something that has been in every CoD game I've played and so far no one has even tried to address this issue.
I say lag. It's the split second or two of lag that can make or break your round in the match. You are either going to have a almost lagless game where everyone can kill anyone else without much trouble, a game where you are in god mod and can absorb bullets and dodge death like no ones business, and a game where you are a second behind everyone else so they see you and shoot you first (even if on your screen you see and shoot first).
GT: Boomer and Tank Pretty darn confident..... I don't want to have to wait till the Microendony XBOXS 4ii comes out another 15 years later....
It doesn't matter how amazing your accuracy is, how well you know the map, hell it doesn't matter that you know someones coming around the corner and you are ready to pre-fire when the game itself doesn't have anything you do count and puts you 1+ second behind people.
When I read about the evils of drinking, I gave up reading.