If you're thinking to write a story for me, or to incorporate my preferences into something you're already making, well, here are my general preferences and the tropes that engage me as a reader (and, obviously, as a writer as well).
I can enjoy a wide variety of experiences; there's not much that turns me off, and even less that triggers me.
Preferred Rating: Mature to Explicit
The thing is, though, that this is my preference because I like Intense Drama, and it's difficult to find intense drama in the lighter-rated fics (Gen or Teen). If you can manage intense and detailed drama in lower ratings, great! I can also enjoy things that aren't intense drama, but that's my preference.
Do not throw established relationships under the bus! You have no idea how irritated I get when a fic tries to make its preferred ship reasonable by pretending that the original ship was somehow less ("I thought I loved Grace, but now I realize that I was merely infatuated; I didn't know what love truly was until I fell in love with you, John!").
Is it so very hard to imagine that a person might love two people at the same time? Or might have truly loved one person, but then moved on and loved another person, without casting a shadow over the original love?
(I personally believe that John's relationship with Jessica was… deeply flawed, but that's not because I just want to throw Jessica under the bus and move on to my preferred ships.)
So if a character is already in some other relationship, then you'll need to provide some justification for moving into a new relationship. Ideally, one that doesn't make the character look like a jerk. "I wish I could be with Grace, but it's too dangerous for her; since that's not possible, I'm willing to engage in another relationship" is not an unreasonable position. (Of course, some of these problems can be easily solved with a threesome: Rinch/Grace is a surprisingly viable relationship.)
I have a decided interest in deep, intimate, Platonic friendships -- particularly between two men. The "David & Jonathan" relationship ("a love stronger than that of/for a woman"), before people reinterpreted that relationship as sexual.
Creating a relationship that is deeply intimate without being sexual or romantic is quite the challenge nowadays, but my favorite pairings have always showcased an emotional rapport, a strong and resilient caring for each other and a willingness to sacrifice on the other's behalf.
Now, many intimate relationships lack a physical component of any significance, but I have a great affection for such nonsexual activities as massages, cuddling, or comforting by touch. (Of course, with canon touch-averse characters such as Harold Finch, you'll have to explain why he accepts the intimacy of touch in your fic.) So, depending on the pairing, you needn't shy from physical connection.
I find it sad that so many people seem to consider friendship to be some "inferior" category of relationship, as though a relationship has to include sex in order to be meaningful. Besides, it's a bit insulting to claim that you can't deeply care for someone or be close to them unless you want to get in their pants.
That said, I read a lot of fics with romantic and/or sexual relationships, so if you'd definitely prefer to portray such a relationship:
At the moment, I prefer reading about male characters, regardless of the sexual component, so my preference is slash over threesome, threesome (MMF or MMM) over het. I've never had much interest in yuri/femslash. Any level of detail is fine (gen through to explicit).
Unusual orientations, either pointed out or implied, are great: asexual, graysexual, demisexual, sapiosexual… is it "monosexual" if you're only attracted to one specific person? Anyway, characters whose sexual preferences fall outside the more common categories are quite enjoyable to read about, and it affects how they respond/react to the idea of sex, as well as how much sexual experience they've had. (I also have a fondness for setups that involve sexual contact outside your own orientation, when it's not treated as the gateway to realizing that you're gay/bi. Situational sexuality, sexual bargains, sex pollen, etc.)
If you want to write consensual sex (happy, emotional, angsty, whatever), don't make it the focus of the tale -- although, depending on the length of the story, a full chapter or two would be nice.
If it's the focus of the tale, I prefer at least DubCon; NonCon and straight-up rape fics are also fine, though I prefer positive endings to even the darkest fics.
I adore the many varieties of Omegaverse (Alpha/Beta/Omega), and Dom/sub verses fascinate me (my introduction to Person of Interest was the fic Dangerous If Unbound by astolat). Similar settings that craft some form of alternative gender balance would be quite welcome.
I'm up for any fantasy, sci-fi, horror, or supernatural AU if it doesn't much change the basic team dynamics. "They solve their mysteries... in SPACE! (or the Wild West, or Feudal Japan, or a world where they're all rodents, etc.)" can be great; "they all went to Hogwarts" (Coffee Shop AUs, ) is much harder for me to appreciate. Though I've been starting to develop an appreciation for Prostitute AUs; they've got more depth than I anticipated.
The character gets taken prisoner, endures the hardships of the captivity, is eventually rescued.
The capture includes either a struggle against physical force (not just threats), or being knocked out, and I do so love the slow surrender to the inevitable as you lose strength/power to keep struggling… whether that's drugs or being tazed, being choked into unconsciousness or briefly drowned or having gotten a blow to the head that slowly brings you under. And I like that slow process being detailed, keeping me in the moment, which is the difference between these two:
The man pressed the rag to Harold’s face, and Harold felt himself slowly going unconscious; he struggled a bit before passing out.
Despite Harold’s struggles, he couldn’t keep the cloth from finding his face. The sickly-sweet scent of chemical citrus filled his nose. His muscles were going weak, giving in, the world going dark around him. As his body relaxed into the man’s grip, but before Harold was quite gone, he felt the man began to hoist him up into the van, and some other set of hands grab him -- but then even the sensation of movement faded away.
If it makes my stomach clench or bottom out (like I’m in a carnival ride), that’s perfect.
The captivity itself should involve fear (especially "not knowing what's coming"), and emotional turmoil, and possibly pain, even torture (which can be mild, moderate, or intense), but nothing all that gory or gross or permanently harmful (some small cuts/scars/burns would be okay).
It should be to a purpose -- the captors want something from their captive -- rather than just being sadists or doing random harm for fun. One possible route would be using drugs that make it harder for the captive to resist orders and/or harder for him to keep his mouth shut about important topics.
I would be okay with a second captive being threatened or hurt to try to get the character to talk. Again, any harm done should be temporary, not permanent or gory.
The rescue needn't be particularly dwelt on (unless you want to detail the section on healing after this trauma) -- but it should assure us that the scenario is over, the character and his allies are free and safe, and that the characters will now have whatever resources they need to recover from this ordeal.
Rating-wise: Give it some leeway to be kinda dark, but without gore or much (if any) swearing; I'm thinking Mature would be about right, but that's flexible.
Ramp up those emotions! I love the cycles of Tension and Release, and the stronger and more nuanced the Tension, the more relief you get from the Release. A good, strong, dramatic plot with a positive ending brings you to the finale with a strong sense of completion; it's satisfying, like a good meal, and relaxing, like the feeling you get after exercising or after getting off a roller coaster.
All the good characters should be well and whole and free -- perhaps with some lingering hints of trauma (physical or mental/emotional), minor scarring, but they're on their feet and back to equilibrium.
Nobody should get away with being nasty; no Karma Houdinis. Bad characters either get redemption arcs (turning them into good characters) or negative ends (imprisonment, death, a fate worse than death, etc.). Make it feel like there's a force that ensures some level of justice, when all is said and done.
I can't stand Cringe Comedy, or its cousin Cringe Drama; material that works on embarrassment or "any second now I'm gonna get caught being where/who I'm not supposed to be" makes me intensely uncomfortable, to the point where sometimes I'll leave the room until the scene is over. I've been known to take a whole day watching a single episode, just by pausing it and doing something else until I can stand to watch a little more, and then pausing it again, and so on.
Comedy in general is less appealing to me than drama or dramedy, but if you're going to work with comedy, go for witty banter, snark, unexpected twists, randomness, and equivocal humor (words/phrases/setups that mean multiple things). I especially like conversations where each side thinks that the other means something else, but where it doesn't rely on them being idiots and not asking obvious questions to clarify the matter at hand.
Redemption Arcs are one of my favorite tropes, actually: Take a villain or antagonist and put them through hell until they're eventually able and willing to join the heroes. Or, reverse that, and give them a reason to join the team first, and then have their experience with the team change them on the inside, until they're no longer just playing a part.
As I mentioned above, kidnapping/capture is one of my favorite plot elements, and can be woven in as part of any larger plot. Bonus points if you make it happen repeatedly.
I don't like gore, gross stuff, or anything scatological (though blood is fine). However, I can enjoy torture that's focused on pain and fear, a minimal amount of physical damage (easily healed later on), trying to bear up under the stress, and perhaps empathy for other captives.