You Don't Even Want to Know and I Can't Describe It!
There should never be a part in your story where a character sees something MIND-NUMBINGLY HORRIFIC and doesn't describe it because they think the reader "doesn't want to know." First off, yes, we do want to know, we're reading your damn story. Second, it's a technique that is obviously used to cover up for the writer's lack of skill. If you could describe it, you would.
Likewise, there should very rarely be something your narrator cannot describe. Sometimes it's effective, if it's earned, if you really have created something that is menacing enough, complex enough, so beyond the scope of human understanding that it strikes the narrator dumb, then it's okay to cop out and say "I can't even begin to describe it." If it's such a generic monster or routine gore, there's no excuse. Again, it almost always points to a lack of skill on the part of the writer.
This is less common, but it still applies: don't have your characters look away from something to get out of describing it. If it's a genuine character moment, it's fine, but if it's laziness, don't do it. If you do it, then you need to describe the aftermath.
If you find yourself writing something you can't describe, there's two solutions:
1. Don't call attention to the fact that you can't describe it. Describe around it. Sketch an outline with a few details and let the reader's imagination fill in the rest. But don't say anything like, "His hair was greasy and scraggly and covered in dirt, his mouth was full of oddly-angled teeth and his eyes, oh God, I can't even begin to describe his eyes."
2. Remove it. Now, if this is a key part of your story that might not be an option, but if it's just another gore scene, if it doesn't add anything new then drop it. It will make your story flow much better.
I Used to be Normal, Guess What I am Now.
Just like you shouldn't open your story with "Have you ever. . ." you shouldn't start it with "I used to be a normal person" and then go on to describe the ways this person was normal. It's been done too, too much and it's not a good opening.
First, it's so easy to show that a character is normal. Start your story with them leaving an office or doing homework or walking a dog. If they're married, write a little exchange between them and their spouse or have them complain about how messy they are. This is a good opportunity to not only SHOW that this person is normal, but to also show us a little bit of who they are. This is much more natural. Most people are normal people, you don't need to go out of your way to disclaim that. You wouldn't start your story with: "Let me make this clear, I walk on two feet. I put one foot in front of the other and this motion moves my body forward and this is how I move." It's a given.
The other thing is now we know the exact trajectory of the story. This normal person will, by the end, no longer be normal. Now, given that it's pretty easy to guess most creepypasta stories will go this way it doesn't do you any favors to include it.
I'm So Awesome and Here's the Story to Prove it
Jeff the Killer is a story that a lot of people point to as having a ridiculously over-powered and skilled main character (Called Mary-Sue/Gary-Stu in some circles). And it's true, for a teenaged suburban kid he's an incredibly skilled fighter and is very strong. The writer makes no attempt to justify this and it makes the whole thing seem very silly.
I would say that's a rather middling example. I've read stories where the main character, also teenaged suburban kids, though that doesn't matter much, who have anime-esque superpowers. There's also characters being expert shots, deadly with any kind of knife or sword, the bravest person on Earth, the coolest dude, the baddest of the bad etc, etc, etc.
This problem is made 100x worse when the story is told in first person. Even if it's not the writer's intent, whenever I read something written in first person where the main character is a teenager and is the MOST AMAZING PERSON IN THE WORLD I automatically assume it's a self-insert. I figure that the writer has the same name as the character, they live in similar places, have similar people they hate. After that, the story just becomes. . .juvenile and sad. Again, it might not ever be the author's intention to put themselves in the story, but that's how it reads.
That's not what really ruins the story, though. What ruins the story is when a skill set appears that we've had no mentions or allusions to. It feels cheap, it's lame, it really draws attention to the fact that your character just became DOUBLE AWESOME!
The bigger problem is that your story just got fucking boring. I don't want to read about a person who is super good at killing monsters or never feels fear or is so cool he should be elected Kid President. Because what's the point? Okay, this person is great, they're going to win. They have no flaws, there's no danger or suspense and it will never, ever, in any way be creepy or scary. The scariest things are the things we can't fight.
To sum up: Justify any amazing skills or knowledge your characters have, don't give your characters super powers, flawed characters are more interesting and the more skilled your character is the more danger you have to place them in.
That last one is a slippery slope. If you want your character to be a super soldier, so you have them battle against an entire army of Rakes, well, that's ridiculous. It could be written well, but it would be much more difficult.
PARENTS JUST DON'T UNDERSTAND
So, so, so, so, so many stories hinge on ridiculously cruel parents. This is a major symptom of a Jeff the Killer knock-off. It's not only found there, though.
There are bad parents in the world. There are some who are downright evil. The thing is when you know that the primary audience for creepypasta is teenagers the evil parents thing becomes a major eye-roller.
The bad parent is usually a crutch for bad writing. It's an easy villain. Parents suck, nobody should be mean to children, it seems like a good formula to create sympathy and focus hate. And that can work, but instead of creating complex characters with motivation and reasons/excuses for why they are like they are, creepypasta writers tend to make parents into mustache twirling villains. Even terrible people have triggers, there has to be some reason for them to explode.
So, if your story parents flies into a rage because their child has an imaginary friend or is just playing, then that's dumb. Again, I'm sure it happens in real life, but fiction, but definition, doesn't have to reflect real life.
Don't have story parents do things just because it's convenient for the story. Justify them, build them up, don't have them just be, "I'M CRAZY AND I'M TELLING YOU THAT YOU CAN'T SEE YOUR BEST FRIEND BECAUSE THAT WILL GIVE YOU A REASON TO MURDER ME!"
And parents rarely stop loving their children. Jeff the Killer is especially stupid in how quickly his mother stops loving him. He becomes disfigured and she's basically like, "No thanks!" If you have a parent make a 180 degree change, you need to justify it and add an appropriate emotional impact.
Fashion Show and Introduction Jamboree!
Thankfully, I don't see this one a lot.
When you're introducing characters, you shouldn't do this:
I went into the forest with my friend Kathy, Rick and Nelly. Kathy was 13-years-old, she loved sports and sledding. She was wearing a bright yellow top, skinny jeans and green sneakers. Rick was a classic bad boy, he was always getting in trouble and he just didn't care. He was wearing a leather jacket, white t-shirt, jeans and boots. Nelly was a real crazy girl, she was always saying random stuff and making faces. She was wearing a purple dress and blue flip-flops.
When you introduce a character, you shouldn't tell us everything about them. Character development is best when it's spread out and when it's inserted naturally. Don't tell us Kathy loves sports, have her talk about having practice later or dress her in sporty clothes. Loading all the information about all your characters in one chunk overloads the reader with information. They're probably not going to remember all the names and traits. A lot of times the details aren't even important. When you build a character's personality through the story it makes it easier to separate inconsequential stuff from things that matter.
And clothes. God damn, some writers think that clothes are the key to solid description. Now, clothes can be great for telling the reader about a character. A person wears a suit, we have built-in notions about the kind of person they are. A person has dirty clothes, we make assumptions. It doesn't work when you have a bunch of generic people. It doesn't build images well, because very few people will sit there and mentally go, "Okay, there's the purple shirt and the jeans and now the shoes." All it does is grind your story to a halt. You can still describe the clothes, but never do it all at once. UNLESS, UNLESS, UNLESS it will tell us something about the character.
Also, news reports about serial killers on the loose will never focus heavily on clothing. They might mention it, but it's not an effective way to spot someone. Because people can change their clothes.