1. Incredibly intelligent characters who lack basic reasoning skills.
There's a draw to having whip-smart characters, but readers get pissed when the aforementioned brilliant characters miss obvious clues or repeatedly make dumb decisions for no other reason than serving the plot. This does raise two interesting questions: 1) How do you write a character who might be smarter than you? [hint: you need to have a mystery that is actually difficult to solve and have your smart character navigate through it reasonably] 2) Why aren't there more books about people who aren't whip smart? How about Tim Riggins from Friday Night Lights solving mysteries? (Billy can help... when he's not getting in the way. Twist: Jason Street turns out to be the killer.) I am, by the way, writing a whole other post about the overpopulation of brilliant people in contemporary fiction. More on that later..
2. Buff characters who never work out.
Buff doesn't grow on trees. You have to schedule a lot of your life around the gym. You book hotels based on whether or not they have a gym. Active people feel uncomfortable if they go several days without being active. Corollary to this: thin women who never seem to work out and eat a lot of junk food because they just happen to have a high metabolism--I think this is a feeble attempt at being girlpower by not wanting to depict a woman who watches what she eats. But if you want to be progressive by not having women want to change their bodies, you don't get to also default to always having thin characters. There are people of all sizes who go to the gym, ones that don't, people who watch what they eat for whatever reason, people who don't. Real women in America, or at least a lot of them, talk and think about food in ways that are deeply fucked up. Some of these women are strong, intelligent, complicated, and interesting. There's nothing wrong with showing that.
3. Scholarships don't exist.
Non-rich teen has no money and/or screws up their chances for a single scholarship by winning the spelling bee/ sheep-shearing contest/ ironic beauty pageant, so now they can't go to college!!!! Their life is ruined! Actually, it's called a FAFSA. You can go to college, but welcome to the world of student debt we all live in.
4. Character is a CEO / high-powered attorney / neurosurgeon but never seems to be working.
This shit takes time. And often times, lots of schooling. Son who inherits company from his father and is a lazy ne'er-do-well--? More believable that Mr. Career awesome who never works.
5. Characters don't use cell phones the way they are actually used.
That is to say, to look up just about anything. Realistically, the introduction of cell phones and the internet in general is a massive game changer in any mystery, horror, or danger type situation. This is related to why I've been disappointed in horror movies in the past 15 or so years: the villains got smarter but the protagonists didn't become more clever.
6. Lame excuses for not telling the authorities/ cops/ parents when a dead body is discovered.
Please provide a reasonable explanation as to why these people wouldn't just tell their parents or call the cops. Remember HBO's The Night Of? If that ever happens to you, don't run away from the dead body, then go back, break in, get your keys, and then try to keep it a secret. He probably should have just stayed exactly where he was, touched nothing, and called the cops.* Lots of people don't actually trust the police, for legit reasons, but too often the excuse is, "but we can't call the cops because we can't!" (*On second thought, if your name is Riz Ahmed, maybe call your dad and a lawyer first, and I guess I would say when a chick wants to play that knife-hand game, it is time to say you're going to the bathroom and run far, far away).
7. Newbies handling guns with amazing accuracy and no fatigue, often holding them with one hand sideways. Guns are heavy and hard to handle and even people who are highly trained professionals miss their targets.
8. Cities devoid of minorities. Actually, this isn't petty but serious. I can't stand when I read a book that takes place in a city I've spent a considerable amount of time in and the actual racial makeup of the city is depicted as wildly off. California has a ton of Asians. The real Beverly Hills High, from Beverly Hills 90210, has a large population of Persians, as does Los Angeles in general. New York City is not just filled with 20-30-something aspiring hipster writers having epiphanies. If you want to write about an all white city, pick one that actually demographically looks that way. Writing and talking about race is super awkward; at least we are at the point in time that we are starting to have the conversation. I admit I'm cautious about bringing up race in my own writing, both because I'm afraid of getting something wrong, but also because even being explicit about it sometimes makes people uncomfortable. I'm working on this.
8. People die for literary sweeps week. Remember on TV when you knew good shit was going to go down because it was sweeps week? Similar to this, I hate how death is always a plot device in books. Someone only dies if it serves some dramatic purpose. In reality, sometimes death is a random wrench thrown into a machine that was originally headed in a different direction. I think this would be a really awesome way to get rid of Frank Underwood in House of Cards. All this intrigue is going on where the journalists are closing in on all the shady stuff he has done. Chess pieces are being moved. Then, in the middle of everything, he up and has a heart attack, or his fucking rowing machine breaks, sending the fan part directly into his face and impaling his brain. Claire becomes president. All Claire all the time!!!
9. "As you know Bob" dialogue in general. NCIS is a perfect example of this. One biologist explaining to another what DNA is. One thing I loved about The Wire: there was absolutely no hand holding.
10. No one ever has their period. Or almost never. And it's too often used for comedy rather than this thing that is there for a huge segment of the population.