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.
Anvilicious: Killing Castro in the first mission (or rather, his double) causes you to get the achievement "Death to Dictators", and the killing shot goes into slow motion while heavenly music plays. It's pretty heavy-handed, especially considering that Castro is a living leader of a modern country. The Cuban government was not pleased when they found out. They even accused the US of trying to do virtually what they failed to do in Real Life.
Heavenly might be taking it a bit far... Castro's definitely portrayed unsympathetically, but everything you do in the game is made to seem shady at best, unethical at worst. The Castro mission was no exception.
On the other hand, Castro is shown to be quite cordial with Dragovich. Castro may have not known how much of a Complete Monster Dragovich was, but still.
Complacent Gaming Syndrome: Treyarch appeased the NUKETOWNNUKETOWNNUKETOWN crowd with a playlist that rotates on that map alone, though it's only available during double-experience weekends on consoles.
Fridge Brilliance: Reznov, why are you so useless in combat since Vorkuta? Because he's not actually there.
Why do enemies keep shouting "VORKUTA!" during combat? That's were you were brainwashed.
Right after Woods sacrifices himself, you hallucinate he's asking you if you're okay, before it turns out it's actually Reznov asking the question. They're both dead.
Wait, if most of the missions are told via Flashback, why can I play as Hudson? because he's the one interrogating Mason.
Foreshadowed in Numbers when you, playing as Hudson, dish out a little bit of "Press X To Torture" on Dr. Clarke.
After Vorkuta, how the hell did Mason sneak back across the Berlin Wall? Dragovich needed him to because of the brainwash, and probably had some strings pulled to facilitate it.
A common complaint about the multiplayer is that lag can make it look like you were killed after you hid behind cover, leading people to call them magic bullets. Remember, this is a game involving the Kennedy Assassination, magic bullets are par for the course.
Fridge Horror: After playing Black Ops, you can't help but despair whenever you finish World At War, knowing that instead of their much-deserved "heroes return", Petrenko will be horribly killed within a few months and Reznov will be thrown into a hellhole gulag for nearly two decades.
Good Bad Bugs: If afflicted by a Nova Gas grenade, the player's character will call out a warning. If they stay in it, they'll call it out again later. Wearing a Tactical Mask makes a player immune to the effects of Nova Gas, thus causing a player using it that decides to stand in the gas whatever reason to be yelling something like "GAS, INCOMING!" to Department of Redundancy Department-levels for their allies to hear and be amused by.
Narm: The Russian Roulette scene in Payback, between Bowman's "communist piece of shit", Mason's sudden accent shift, and Woods' overacting — "YOU CAN'T KILL ME!".
Nightmare Fuel: Reznov frequently shows up to help Mason in battle. Over the course of the game, one cannot help but get the feeling that something is wrong with how convenient his appearances are. This creepy, unsettling feeling reaches a peak at the end of "Rebirth" and during "Revelations," where it is revealed he was a figment of Mason's brainwashed mind all along. There are also the implications that JFK assassin Lee Harvey Oswald was also a Soviet sleeper agent ("Oswald...compromised...") and that Mason eventually kills JFK in the very last moments of the final cutscene.
How about Reznov reverse brainwashing Mason to seek revenge on his enemies? Couple with the moral choices in World at War...'
Looking at intel can be exceptionally creepy, because all the ambient noise is erased from the room, as well as the music, except for this ongoing, creepy, wheezing noise like deep breathing or some kind of ventilation machine. It begins to wear on you after about two minutes and becomes deeply unnerving after five. It's never specified what it is, either...
"Revelations." All of it. Mason experiences a numbers-addled Freak Out as he staggers through the halls of the facility, experiencing flashbacks and suffering immense pain as the numbers affect him. The imagery, combined with the dissonant, echoing effect of the sound clips, is truly terrifying. It ceases at the very end, however, with Hudson doing his Glasses Pullwith a quick, action-packed guitar riff playing.
Rewatch Bonus: The little details about Reznov stand out much more on a second playthrough. Why is he continuously going on separate missions with Mason's MACV-SOG unit? Why is he looking healthy and squared away like one of the American Marines, even wearing an American uniform? The answer to the first question is because he's not really there. The answer to the second question is likely because Mason is replacing an actual American on each mission with Reznov in his mind. It would be Foreshadowing, except it's cleverly hidden by Weaver's presence; because Mason describes him as being absolutely trustworthy despite being Russian in a short throwaway line, we assume the unit has some sort of policy that lets defectors serve with them. The first time through, the player assumes it's just a weak excuse to keep Reznov around for more of the game, and since most players like him, it's not even thought of as a problem. Even Reznov's appearance in the tunnel isn't true Foreshadowing, because it's made to look like Mason's buddy is chastising him for being startled, hiding the fact that he's just seen Mason talk to someone who isn't there.
Additionally, Mason's fanatical need to kill Dragovich and the others can easily be explained on a first playthrough by revenge and the fact that he was directly ordered to by John F. Kennedy himself. On a second run, you can see the compulsion programmed in by Reznov.
Lag compensation. This is the game that started it all. With all the misplaced fan complaints about lag ruining their games, Treyarch decided to redo the netcode from the ground up. Unfortunately, the changes they made came with some rather negative effects on the gameplay experience. They added in something that attempted to "equalize" everyone's connection when the game starts, but this made the experience comparable to having a host from across the world. In addition, host advantage, which was virtually unnoticeable in earlier games, became very noticeable due to the changes. Having host was practically a requirement to use killstreaks such as the Chopper Gunner because, more often than not, if you're not the host you won't be able to kill anyone with it.
Respawn point placement — particularly on the Firing Range and Nuketown maps when playing either Capture the Flag or Demolition. Respawn points for each team is restricted to a very small area of these maps, and it's not too difficult for a team of experienced players to trap the other team into a corner, racking up kills as quickly as the other team's players pop back to life. Worse still, if the dominant team has both a Chopper Gunner overhead and attack dogs on the ground, that basically tells players on the weaker team to put their controllers down for the next sixty seconds.
Specifically, respawning enemies with various heavy weapons, that you have to stand up in front of three separate times to pass a certain evil check point.
Infinite enemies also appear at the very end of "Executive Order". Worst. Hallway. Ever.
The Untwist: It's actually not that hard to figure out that Reznov was dead the whole time before the big reveal, even on the first playthrough, if you pay enough attention.
Win Back The Crowd: After the dedicated server brouhaha with Modern Warfare 2, Black Ops came to PC with dedicated servers and a server browser again. Not to mention the return of everybody's favorite gamemode from World at War. Black Ops 2 is also having this effect from the moment of its trailer's debut. Less linear gameplay and different endings based on how you do? The cries of stagnation are finished.