The whole door opening exchange in "No Fighting in the War Room!" in 4.
Gaz: Got it, door's coming on line now.
Griggs: Oh you gotta be shittin' me!
Price: Gaz, can't you make it open faster?
Gaz: Negative, sir, but you can try pullin' it if it'll make you feel better.
Price: ...Cheeky bastard.
In "Heat," Gaz's frustration with the helicopter pilot.
Pilot: Bravo-Six, this is Gryphon Two-Seven. We've just crossed Azerbaijani airspace. E.T.A. is four minutes. Be ready for pickup. Bravo-Six, the LZ is too hot! We cannot land at the farm! I repeat, we CANNOT land at the farm! We're picking up SAM sites all over these mountains!
Gaz: Oh, that's just great! Now, where are they gonna land now?
Pilot: Bravo-Six, we're getting a lot of enemy radar signatures, we'll try to land closer to the bottom of the hill to avoid a lock-on.
Gaz: Oh, he's gotta be takin' the piss! We just busted our arses to get to this LZ, and now they want us to go all the way back down?!
"Oi, Suzy!" *thwack*
As a Call Back to Worm's line in "The Gulag" in Modern Warfare 2:
Soap: Who the bloody hell is Yuri?
When Wallcroft and Burns are advancing up the escalator in "Mind The Gap" a Russian rolls a grenade down the steps. It is pretty easy to avoid, but Wallcroft's over-the-top reaction is hilarious. It doubles as a Call Back to the first entry.
Wallcroft: CHEEKY BASTARDS!
Pretty much the entire Veteran/Noob commercial for Modern Warfare 3.
In "Wolverines", you can set up the sentry gun on the roof of the restaurant to shoot at Russians that are coming up from an access hatch. However, just like when they rope out of a helicopter, if you shoot them on a ladder they skip the death animation and become limp and weightless ragdolls. If they climb up onto the roof still in the ladder animation when the sentry gun shoots them, there is a good chance that a Russian soldier will be blasted off, cartwheeling hundreds of feet into the air...becoming a little black dot on the horizon.
A bit of unintentional humor in a mission where you have to protect The Russian President from an assassination attempt. You get him safely to the helicopter, and open the door only to be shot by Makarov, in a fashion similar to how he offed Joseph Allen in MW2. It became a bit of a mood disruptor when one started to half-expect that to happen every time someone went to open a door.
In the Manhattan Stock Exchange, you see that everyone's stocks are dropping (obviously, since there is a battle in New York going on RIGHT NOW)... except for Infinity Ward, Treyarch, Activision, and Sledgehammer Games. I find that oddly hilarious.
Here's one from "Persona non Grata". "Yuri! We need to get Soap to the chopper!" *chopper crashes into the hospital room*.
When entering the semi-ruined house on Brookmere Road, one of the first things you will see is a Russian soldier examining the contents of the fridge.
Soap: "We'll stick out like bollocks on a bulldog."
Meta-example : 2011 game of the year award on every game review websites are tense with Battlefield 3 and Modern Warfare 3 just asking for a final showdown we're all waiting for. Gamespot's best shooting game of 2011 award goes to Gears of War 3.