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.
YMMV: Rainbow Six
Accidental Aesop: Rainbow Six was intended as a screed against ecoterrorism and animal rights extremists. However, Clancy had to put the ecoterrorists in charge of a huge megacorporation to make them credible villains, which is pretty unrealistic. The novel can instead be taken as a tract against unchecked corporate power.
Magnificent Bastard: Popov, for setting things up so that if the terrorists he sponsors get defeated, he keeps the money he would have paid them. Also, at the end, he backstabs his Horizon backers and spills the beans on them to Rainbow, earning himself a full pardon. Then he uses his money to buy up the ranch of one of the deceased Horizon executives, since he knows that land there is rich in gold.
They Wasted a Perfectly Good Plot: Multiple chapters are spent building up the ecoterrorist's American headquarters, and highlighting all the things that would make it a very tough nut to crack. It's played up as the Kilimanjaro of forced entry scenarios, a hostage taker's wet dream. And then the ecoterrorists abandon the facility and flee to a smaller outpost in the jungle. Which they then leave, in order to engage in a jungle shootout with a team made up of the deadliest special forces operators in the world, which even the protagonists recognize as a painfully lopsided scenario.
Anticlimax Boss: Bastian Vanderwaal in Lockdown and Irena Morales in Vegas. Vegas 2 ends with a one-on-one quickdraw between you and Big Bad Gabriel Nowak, which might have worked, except he gives a long Motive Rant before either of you draw your pistols, and once you do he continues to rant on while you have your pistol aimed right at his head. Miguel Cabrero can be something of a Kaizo Trap though; he dropped his gun earlier, but he has another one in his holster and will pull it out once you've finished talking to him.
Right before Nowak gets his head blown off, he sicced an attack helicopter on you. So yeah, the unfair quickdraw was more or less payback. To make it even more ridiculous, prior to going to face Nowak, Bishop orders the rest of his\her team to hold position. So, Bishop basically wanted a one-on-one confrontation, and got an assault helicopter for his trouble.
John Brightling in Shadow Vanguard. You kill all his mooks, blow up his door, and then a cutscene shows you killing his last two bodyguards before he is captured and gives a potential sequel hook.
In the first three games, Elite difficulty makes all enemies into this (they most frequently get insta-death shots, have incredible accuracy and range, automatically aim without directly facing you, etc.) In Rogue Spear and beyond, if a crouched enemy sees you, they WILL immediately headshot you.
Any stealth level in the original games. To wit: it's Metal Gear on European Extreme. In first person, which just makes it harder.
Older Than They Think: While Rainbow Six was the first to do it in 3D first person, the British DOS game Deadline featured similar planning-based tactical real-time anti-terrorist gameplay a few years earlier, only from an XCOM-like isometric third-person perspective.
The fundamental concept originated with Ariolasoft's forgotten They Stole a Million, a heist game from 1986 in which the player carried out a series of robberies, first selecting team members and then planning their movements with the aid of stolen blueprints before acting them out in real time. With the exception of gunplay the concept and many of the details were identical.
Scrappy Level: Every single damn level in Vegas 1. Low hit points plus distant saving locations equals an annoying, hard slog as you constantly have to clear the same levels over and over as you die. Vegas 2 is better about this, but not by much.
That One Level: The refinery level in Vegas 2 which you don't have your teammates to back you up.
The final level in the first game. It's really long, and there are a lot of blind corners with enemies hiding around them.
Villain Decay: Gabriel Nowak, the Big Bad of the Rainbow Six: Vegas series, goes through this rapidly in the second game; as soon as he's revealed to be the mastermind behind the entire terrorist plot, his previously mysterious master plan and motives are rapidly revealed to be nothing more than a hissy fit thrown because he was the team screw-up.