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.
Complete Monster: Niska becomes an even more extreme one when he orders his men to rape River, as a "favor" since she is about to "die a virgin".
Lt. Womack, already a corrupt scumbag in the series, turns plain vicious in Silver, where he cold-bloodedly murders most of his associates and suppliers to eliminate the evidence of his organ smuggling operation, and then forces Mal to destroy the last remaining lab rather than risk himself.
And at the end of the arc, Womack kills one of his own men in cold blood so he can escape from his crashing ship. Fun guy.
A possible example is Inducer One-One-Nine, aka Kathryn Wade who killed nearly a thousand people with her psychic abilities, using them as shields or minions or, in one vicious case, as distractions against her pursuers, and uses Mind Rape quite liberally, up to an including River. There is quite a bit of disagreement among the readers as to whether One-One-Nine is a monstrous figure or a tragic figure, considering she was nine years old when she escaped and was mentally unstable to the point of sociopathy by her stay at the Academy.
The "Mongol" of the Six Rifles. He tortures a man he's been sent to kill with a bit of Dissonant Serenity, and at least one other member of the Six contemplates having to kill him eventually if he goes out of control. Later on, when he captures the young Jayne in a flashback, he goes onto a lengthy monologue about torture, where he offhandedly mentions that he is also a rapist and talks about how much he loves inflicting terror on his victims.
"Home," she said, and she started crying into his neck, her body shivering with sobs. "I want to go home, Jayne." "Me too," he hissed, and started out the door with the featherweight of little crazy person on his back. Pistol leading, Jayne Cobb stepped out of that hell-chamber, and started their escape. He was gonna get her out of here, safe and sound, even if the whole gorram Alliance stood in his way.
Later, River confronting Colonel Dannet Partway through she realizes she can't kill him because she simply can't bring herself to hate him, then she has another bout of emotional madness, and he ends up comforting her. The entire confrontation ends with her forgiving him.
Les Yay: the brief scene between River and Grace in "Mosiac" is nothing but.
Moral Event Horizon: When Katie was outed as an Inducer, many of the readers were initially sympathetic to her. Then, one chapter later, she made Pherson shoot and critically, possibly even mortally, wound Mal and then she mindraped and kidnapped River, at which point the general opinion shifted to " She needs to die.
Nightmare Fuel: More than one reviewer has said that the "Inducer" psychics are outright terrifying. And it's justified: there is absolutely no defense against their effects. Not even knowing they're attacking your mind is a defense against them. As one character puts it, knowing an Inducer is attacking you is "like knowing the name of the person beating you to death with a sledgehammer." Not to mention that their effects are ongoing; once an Inducer creates an emotional response, it apparently self-perpetuates, i.e. making a person angry at someone causes that person to get angry everytime they see that person, simply by association, until it becomes outright hatred. And if an Inducer gives up on subtlety and decides to make a full-on attack to bend someone to their will it breaks the victim's mind and drives them insane. Very powerful, and very scary.
One of the reviewers even said the concept quite literally gave them nightmares.
It's hard to tell whether it mitigates the Nightmare Fuel or just makes it worse, but Katie says that Inducers can't actually implant emotions or feelings but must work with what's already present. River manipulated into lusting after Jayne? Those feelings were already there. Friends or allies manipulated into killing each other? At some level that hatred already exists!
Shipping: All of the canon ships are present, including Wash/Zoe, Simon/Kaylee, and Mal and Inara's UST. The author has confirmed that there will be a pairing involving River, though with whom is up in the air.
That last bit has been confirmed by the "Second Interlude." Echo was apparently used as a means to stabilize River's emotions to keep her from committing suicide, and he's apparently been sent after her since direct attempts to recover her have failed.
River and Jayne also gets a lot of play, particularly after Katie manipulates River into practically attacking him out of lust. And especially after it's revealed that Inducers can only work with feelings that already exist.
Tear Jerker: Left and right, mostly relating to River. The poor girl goes through a lot in this story.
The Woobie: River. Yes, she's already a Woobie, but this story just amplifies it up to eleven. Aside from suffering multiple mental breakdowns, she is also tortured and nearly raped by Niska's men, before being beaten up on by the Hands of Blue. Then she suffers another psychotic break that ends with Mal having to knock her unconscious.Then she is attacked, wounded, paralyzed, and again nearly raped by Reavers.
Stoic Woobie: Mal most definitely qualifies as this by the end of "Mosaic." Zoe becomes one as well at the end of "Hunt."