Fantasy:
Giant worldbuilding essays. I love to write them, don't get me wrong, but I recognize that they aren't the same thing as a story, and I acknowledge that the average reader would have very little use for them. I wish more fantasy writers would also acknowledge that... 9_9
'Chosen Ones', as in, "someone is supposed to do X, Y, and Z, and we've picked you to be that someone, so get to training!" The concept just bores me; I can't get invested in it or take it seriously. I have created exactly one Chosen One MC in the modern day, and her story's been DOA for 2 years because I just have no idea how to write it (even though it's a parody! o_o)
The Big Quest To Defeat the Ultimate Evil-type plots. I guess I don't hate seeing them, but I don't like to write them and I prefer stories that also do not use them. I just feel like conceptualizing 'evil' as something with a singular source that can be found and eradicated is a bit...childish.
Sci-Fi:
Aggressive technobabble. Again, don't get me wrong-- I love technobabble; although I try to restrain myself I'm probably guilty of this exact thing. XD But I feel like I get a pass because I'm a scientist; I tend to know what I'm talking about or at least how to simplify it. Many sci-fi authors...don't. ^^; And if you stop to think about the terms they're throwing around it quickly becomes obvious that it's just thematic nonsense.
Robots as disposable sidekicks that can be abused and tormented for fun-- HATE HATE HATE HATE HATE IF YOU DO THIS I'M MENTALLY SCRATCHING AND CLAWING YOUR FACE RIGHT NOW
...Not only do I find the "oh, they don't have feelings" excuse disgusting, I think this trope kinda goes against human nature. We love to anthropomorphize things; long before 'robot' was a concept we gave cute pet names to boats and weapons and even buildings, and tried to love and care for them accordingly.
Just in general, I want more sci-fi stories about recognizing the value in non-human objects AND creatures, and less promoting sick outdated ideas of 'supremacy' from the early days of the genre. T_T
Science vs. Ethics-type plots, as if the two concepts can't coexist. Good science always takes ethics into account, and scientists in general are not emotionless data-gathering shells of human beings (seriously, this portrayal needs to stop. It's just sad at this point).
I do like Ethical vs. Unethical scientist conflicts, though. More of that, please~
'ToughGuy McManlyMan' MCs. ^^; I don't even know why this is a trend...maybe because that type of protagonist was popular in the 80s when sci-fi started to get really big...? Anyway, I'm just...not interested
Mad scientists specifically as people who do whatever-the-heck and have lost all contact with reality. It's just too unrealistic for me...you know what usually happens when you goof around in the lab with no plan and no idea what you're doing? You accomplish very little, and then you die. ^^ There are plenty of ways to write a 'mad' scientist who's still competent enough to get things done without relying solely on luck and coincidence.