The online is horrid for multiple reasons.
You get 50 points for killing someone with one shot. What the forget?
Wow, you killed someone with 1 shot, let me give you 10 levels! Hurrr
At the beginning of each game, everyone runs around throwing semtex's like loving idiots playing baseball with grenades.
Nobody uses any tactical skill. In Call of Duty 4, we talk as if we really are in the army. "Me and John will flank right and head to grandmas house. You two run down charley's isle and use those cars as cover. Go!"
We actually do that, and many of the earler Call of Dutys did as well, such as 2 and 3.