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.
Crazy Awesome: Vaas. Did he ever tell you the definition of insanity?
Complete Monster: Bigger Bad Hoyt Volker is the boss of the biggest slave-trading ring in the South-Pacific. He is described as the root of all the evil happening on the island, and is responsible for manipulating Vaas with drugs into becoming a madman. In his first appearance, he gleefully forces a small group of innocent civilians to run across an active minefield. The second time we see him, he burns one of his own men alive and tells the rest of his privateers to kill any island native they see on sight. Finally, during a poker game, he kills Jason's friend Sam and threatens to cut off one of Jason's fingers every time he loses a round, a threat that he makes good on. Volker is an utterly psychotic sadist without a single redeeming quality.
Heavy pirates can take a tremendous beating from conventional weaponry, and are nearly immune to small arms. But even worse are the pyromaniacs; heavily armored privateers who will burn everything in sight with their flamethrowers and soak up absurd amounts of damage, shrugging off grenade and C4 explosions and openly laughing in your face when you fire upon them. Only a close-range blast from an RPG or the AMR is guaranteed to kill them in one hit.
Snipers can and will use their deadly accuracy to put devastating rounds into you, given even the slightest chance. Even a tiny gap between trees will invite pinpoint fire. Unfortunately for them, if you've got a sniper rifle, the reverse is also true.
Enemies with RPGs (especially armored ones) can be a pain the ass, as the rockets can give you splash damage while you're in cover and leave you vulnerable for other enemies to gang up on.
Did you think you were safe from the sharks in the rivers? Hah. Crocodiles are arguably worse, since it's incredibly hard to see them with the camera, and due to the rivers not having lots of boats, you're very open to attacks from them. The worst thing is that crocodiles are never used for crafting, making them worthless to hunters. It helps that they've one of the scariest Jump Scares in the game. About the only good thing about them is that you can, at least, get into a QTE with them to instakill them - sharks require a boat or some range.
Tigers. Stealthy, fast, and can eat multiple assault rifle magazines without stopping. Tigers are the #1 cause of random encounter deaths in the game. Leopards are almost as bad, being smaller but not quite as deadly. Bears also take several clips to bring down and can down you quickly if you're not careful. Cassowaries can also be deadly if they get in close, simply due to their raw speed. Dingos hunt in large numbers and can be very stealthy in tall grass. Ditto for komodo dragons.
Water Buffalo may look friendly, and they don't get set off as easily as any of the other predators, but make no mistake: bullets have less stopping power against them than any tiger, and they are more than happy to kill you to death in seconds. There's a reason they get marked as 'predator' when you scout them.
On the other hand, Car Fu still beats any of them, and, if you've been destroying outposts, the Shredder makes quick work of anything in your path.
Disappointing Last Level/Ending Fatigue: The majority of the game leads to the confrontation between Vaas and Jason, which occurs three-quarters of the way through the main plot. The rest of the campaign focuses on assassinating Hoyt, Vaas's boss, who is much less colorful and lacks the motifs Vaas had that made him resonate with Jason, and the player.
If you think about it, though, that's kind of the point: Jason has a clear target in Vaas, who's been antagonizing him since he was initially captured and killed his brother Grant and, he thinks, Riley. The second part of the game, after Vaas is dead and Jason puts Hoyt in his sights, the novelty of revenge has worn off. Things are blasÚ and boring, there's no new skills to get, there's just the new enemies and they aren't much challenge because you're a Bad Ass; that's the point. Jason still wants to kill Hoyt, but it's no longer new and interesting and it's reflected in the gameplay. The climax of the game is the fight against Vaas, just like the first time you do drugs; it's never as good as the first time. By the time you get to the second island, the rush is starting to wear off, the drugs aren't working as well, and suddenly...you're looking for another hit that's as good as the first. Whether you take it or not depends on whether you can realize your addiction and try to overcome it, or revel it in.
Draco in Leather Pants: Vaas gets this big time. Never mind that he is an unrepentant slaver and mass murderer, his entertaining personality and hints at a sympathetic backstory have won him legions of fangirls.
Goddamn Bats: Dogs. They come in packs, and they're one of the fastest animal types in the game.
While not quite as fast or aggressive, boar can be annoying due to their willingness to ram anything that moves to death.
While not very dangerous on their own, pirate patrol cars can easily screw up any carefully planned stealth assault on an outpost if they decide to come through and snoop around at the wrong time, which they have a nasty habit of doing.
He's Just Hiding: There are some around the internet who believe Vaas is not really dead. Evidence for this is largely based on how the final fight between Vaas and Jason is a Mind Screw from Jason's perspective, and when Jason makes it back to reality he never sees Vaas' corpse.
Memetic Mutation: "Did I ever tell you the definition of insanity?", has been one since before the game even came out. It has taken a double meaning in the Youtube community who like to joke that Vaas is secretly taking a jab at games in the military first-person shooter genre (such as Call of Duty and Battlefield) and the players who continually buy the "same game".
Moral Event Horizon: Jason possibly crosses it if he elects to kill his friends and brother at the end. He comes perilously close throughout the rest of the game, but the good ending is his redemption.
That One Achievement: "Fully Inked", unlocked by learning all the skills in all three categories. Not only is this a massive time sink, but it can actually be rendered Unwinnable because one of the skills requires you to clear at least two Outposts without a single guard seeing you.
Unless, of course, you find the selection buried deep in the Options menu to reset every outpost, thereby undoing almost all of your non-story-related work.
Just because a region of the map has been cleared of pirate outposts doesn't mean that you're not going to run into random pirate patrols.
No matter where you are, tigers, leopards, komodos, or dogs can show up, and often your only warning is when they bite a chunk out of your arm or leg. The indicators on the map of where they can be found are just a warning that you'll see them more often in those parts of the island.
The fundamental nature of the game's themes about insanity, the subtle supernatural underpinnings, and Willis' warning that the jungle drives people crazy bit-by-bit are a potent form of psychological paranoia. Is it the stress of what's happening that is driving Jason further into his Blood Knight tendency, or is it something more?