Actor Allusion: "The Vet and the N00b" trailer is both this and a Call Back. Sam Worthington plays the "Vet," after previously appearing as the commando Alex Mason in Call of Duty: Black Ops. The commercial also calls back to the Black Ops advertising twice. First, with the tagline "There's a soldier in all of us." Second, having NBA star Dwight Howard appear toting an M16 with a grenade launcher. NBA star Kobe Bryant appeared toting the same gun in ads for Black Ops.
Banned In Russia: Modern Warfare 2 was banned for a while, though the game is now available (apparently with some cut content), including from Russia's biggest online retailer, Ozon.ru.
Beam Me Up, Scotty!: The famous line "Fifty thousand people used to live here... now it's a ghost town." was not spoken by MacMillan. It was actually spoken by Gaz in the intro. MacMillan's line in the game is "Fifty thousand people used to live in this city. Now it's a ghost town... I've never seen anything like it."
Oddly, MacMillan DOES say that line in the Wii port of the first game.
There's a form of this in-series: part of Zakhaev's speech from "Ultimatum" in 1 is reproduced in the introduction for 2 - "My son's blood, on their hands." The original speech didn't include the word "son".
Cash Cow Franchise: In spades. 2 had the largest first-day gross in entertainment history. Not just video games — all of entertainment, including movies and music.
Which was then beaten by Call of Duty: Black Ops. Then that record was beaten by Modern Warfare 3, which managed to smash MW2's record by over a hundred million dollars. Say what you like about the quality of the games, they certainly can sell them.
The developers certainly said they would want to continuously make more games even after Modern Warfare 3 in a live interview. And they certainly did.
Curtis "50 Cent" Jackson did some voice acting for the game (as the multiplayer announcer for the Navy SEALs). Cue Fan Dumb declaring they would not buy the game.
Soap MacTavish is played by Kevin McKidd. In what would be a somewhat recursive instance, McKidd has stated that Infinity Ward expressed interest in having him play Soap in a movie adaptation, but since the movie is still hypothetical, it's not a certainty.
Craig Fairbrass voices three different characters in the series' history (Gaz, Ghost and Wallcroft in that order).
Waraabe, and by proxy the African Militia announcer, is Ike Dubaku
Will GOB Arnett also lent his uber-husky tones, having publicly professed his love for the series. Damned if anyone can tell who he is from the morass of gruff and husky voices that proliferate the soundtrack.
Meaningful Release Date: November 10th, 2009 for Modern Warfare 2, which was advertised in the trailers as "11-10-09", with the numbers blinking on in sequence like the countdown before an explosion.
Name's the Same: One of the randomly-generated names for the FSO agents in the MW3 level "Turbulence" is, minus a single I, the exact same as that of World at War's Russian Player Character.
There's also a Pvt. Allen in "The Bog" in Modern Warfare 1.
Prop Recycling: Closer looks can reveal where Infinity Ward re-used various models to fill out levels. Of note are crashed Technicals in Washington D.C. (oh, those crazy Virginia rednecks), and American police cars on the tarmac at the Russian airport.
Hidden in plain sight during Modern Warfare 3's campaign: There's a warehouse that you fight through which is a literal shot-for-shot recycle of a warehouse from CoD4's multiplayer; even the crates are the same.
The Other Darrin: Nikolai, MacMillan and Kamarov all get new voice actors for 3.
The Red Stapler: Many manufacturers of airsoft replicas have become absolutely drained because of these games. KWA, a former specialist in gas-powered pistols, has stated that their entire stock of Beretta 93R pistols flew out the door in record time, with the reason "thanks to MW2, these things go quick like hotcakes."
Troubled Production: The long-brewing conflict between Activision and lead elements of Infinity Ward finally came to a head during the development of Modern Warfare 3. A large portion of Infinity Ward's staff ended up leaving or being forced out, forcing Activision to bring in Sledgehammer Games to finish development of the title. Famously, one of the disgruntled employees leaked the complete plot of the single player campaign almost a year before release, which was widely published by the gaming press. This, on top of a lot of Infinity Ward leaving during the LAST game...
Urban Legend of Zelda - "Sniper Frog" is a running meme within the MW2 community, after an unsourced GameFAQs post claimed that spfg.exe was a program running in the background which essentially doled out luck to players who weren't doing well and handicapped those who were.
What Could Have Been: In spring of 2011, a lot of info about Modern Warfare 3 was leaked. including concept art and the game's plot (You can read it here). While most of the plot is similar to what made it into the final product, there are several interesting differences such as a mission set in the Kremlin and Sandman participating in the final mission to kill Makarov.