These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
The guys who hate World At War, Big Red One, 3, Black Ops and its sequel because they weren't done by Infinity Ward. Many fans also hate the Juggernaut perk and/or rifle grenades, and the MP-40 in World At War is given similar views.
Complaining about Shows You Don't Watch: Anyone who accuses this series of being jingoistic, pro-American, War Is Glorious screed, which ignores the fact that America gets brutally beaten over two games, a lot of American characters are killed, the underlying War Is Hell theme, that the two main characters are British, and that the Big Bad of Modern Warfare 2 is an American general.
Contested Sequel: World At War attracts a lot of flak for various reasons (unfair respawns in single, Darker and Edgier, going back to World War 2, being by Treyarch, etc). 3 does likewise, largely for pointless motion sensor gimmicks, rather dull graphics and abandoning the "major historic battles" motif in favour of being set in a series of seemingly random towns that even the French have probably never heard of.
Critical Dissonance: Boy is it ever. The games sell very well on the market. However, given the way most people treat the game, you'd wonder where on earth these millions of Call of Duty players are coming from. Hell, even the actual playerbase spends a good quarter of their online playtime claiming either the game they're currently playing or every one of its sequels are terrible.
Fan Nickname: Quite a few people who discovered the original, United Offensive and Call of Duty 2 after playing the next-gen games are now calling them "Call of Duty Classic". The first game was released with this title as a Downloadable game after Modern Warfare 2's release.
The classic CoD gamebreaker is the PPSh-41; in the original, it had a huge 71-round magazine along with very friendly and predictable recoil and an excellent iron sight. Almost every later game tried to Nerf it somehow. Submachine guns in general have always been overpowered in the World War II games.
CoD 1 also had the scoped Mosin-Nagant; while every other scoped rifle had to reload rounds one at a time, the Mosin-Nagant used a stripper clip even when scoped, giving Soviet snipers a huge advantage in multiplayer.
Apart from the other broken scoped rifle, of course. The FG42 in the original game was ridiculous; a selectable semi/full-auto high-damage weapon with an extremely accurate scope, it was a true do-everything gun with no meaningful weaknesses whatsoever.
The rifle grenades in World at War. They're pretty much accurate rocket launchers. If one uses them, they will be hated.
Any form of grenade or rocket launcher is usually decried as a "noob tube". It didn't help that one of the default classes that one had to pick in Cod 4 to unlock Create A C Class had an M16 with grenade launcher attachment.
Genius Bonus: The Dies Irae from Mozart's Requiem plays for a bit during the Soviet invasion of Berlin in "World at War". "Day of Wrath", indeed!
Growing the Beard: Combined with the whole Infinity Ward versus Activision stuff after Modern Warfare 2, general sentiment following the release of Call of Duty: Black Ops 2 is that Treyarch has gone from the makers of "off" Call of Duty games to the new makers of "on" Call of Duty games.
Holy Shit Quotient: The Zombies mode in Black Ops. It begins with Secretary of Defense Robert McNamara, Fidel Castro, John F. Kennedy, and Richard Nixon having a conference after the Cuban Missile Crisis. Then, zombies break into the Pentagon.
Kennedy: Zombies. Gentlemen, in times like these, our capacity to retaliate must be, and has to be, massive to deter all forms of aggression.
(Kennedy opens a secret panel of the conference wall, revealing a stockade of weapons)
Internet Backdraft: Modern Warfare 2. PC gamers were not happy with the lack of dedicated servers, and everyone else agreed that the many Game Breaker bugs left in the final product should have been found and dealt with during playtesting. The level "No Russian" is also extremely controversial. Critics and fans alike seem to fall in two camps - 1) those who think the level was a sadistic and somewhat tasteless attempt to stir up controversy and 2) those who find it gut-wrenching and horrifying but accept that this was most likely Infinity Ward's intent in the first place. There is also a far more frightening (and unsettlingly large) portion of the fandom who actually think the level is cool and gleefully slaughter as many unarmed civilians as possible.
In Modern Warfare 2, many. Makarov for mowing down an airport full of innocent civilians and blaming pinning it on the U.S.; the "good" Russian army vowing to kill 1,000 Americans for every dead Russian civilian in the airport (and they go about this gleefully by shooting down civilian evac choppers); and Shepherd for orchestrating it all because, he claimed, there weren't enough patriots in America.
Black Ops: In case you didn't catch on that Dragovich was a bad guy, the part when he locks up Dimitri Petrenko, the hero from World at War, and exposes him to a horrific flesh-melting chemical weapon just as a test should leave no doubt at all.
No Such Thing as Bad Publicity: No matter how many people hate the game series, it sill manages to sell millions of copies. Whatever the developers are doing, seems they're doing something right...
Older Than They Think: Treyarch's developing games (they made 3, and absorbed the team that created United Offensive after co-developing Big Red One with them) and the yearly release cycle has been around from the very beginning.
Not to mention the heavy use of Decoy Protagonist, although no playable characters actually die until Modern Warfare.
United Offensive was the first game in the series with a Sprint Meter; it was not until Call of Duty 4 that it became ubiquitous in the series.
Sequel Displacement: A vast majority of the fanbase have never played the original game, United Offensive or even Call of Duty 2, largely because the current Call of Duty fanbase consists of Xbox 360 and PS3 gamers, and the early games were PC exclusives before being ported to consoles (with the exception of United Offensive, which still has not seen a console release).
Silent Majority: Judging by the sales figures and how most people react about Call of Duty, this is a very good example of this trope. There are obviously thousands of people who think these games are very good, yet feel no need to scream about it on the internet. (or maybe they're too busy playing the game to scream about it.)
So Bad, It's Good: Listening to the non-American multiplayer announcers in World at War is hilarious. Getting a 7-kill streak is even more rewarding when you get to listen to Gary Oldman Sergeant Reznov yell out "Unleash the dogs!"
"Comrade Sniper", the desert tank missions, and "The Silo" in the second game, especially on Veteran difficulty.
The second half of the Foy mission in United Offensive comes to mind. As if sniping tankbusters while EVERYONE IS SHOOTING AT YOU isn't enough, the final part of the mission is just as brutal.
The very last sections of both the American and Russian campaigns of United Offensive force you to wait for German tanks to show up and be destroyed, while soldiers respawn infinitely. In the American mission, they are so frequent you may not even get enough time to reload your Panzerschreck; in the Russian one there are also soldiers firing at you from afar, and a Stuka doing frequent bomb runs - it may even destroy the artillery piece you can use and force further exposition to scrounge for Panzerfausts around.