Any time I get sucked into a community, I invariably get sucked into the writing side of the community. At various points over the years I have written stories based on Dungeons and Dragons, Star Trek, The Lion King, Furry, and My Little Pony. Since I consider myself to be a moderately better-than-average writer, I will sometimes roll up my sleeves and jump in to help other writers who are still learning the craft. This can take the form of giving helpful critique, all the way to actually doing rudimentary editing for somebody if they are especially receptive to help, and are taking the assistance to heart.
I have my biases, but I try to avoid steering people toward the way that I would write something, and just stick to steering them away from stylistic pitfalls and rookie mistakes.
One mistake that is surprisingly common among beginning writers is changing tense during the story. Their narration swings between past and present tense without it being relevant to the story. Most writers seem to be surprised when I point it out to them, often claiming that I was the first to notice it. How could one not notice? Another common mistake is switching speakers in mid-paragraph, often without changing attribution. Maybe the rules have changed since I was taught, but I learned that any time the speaker changed, you started a new paragraph. One of the benefits of this is that even if you did not attribute the new speaker, the paragraph break gives a clue that it may have changed. Finally, there are writers who switch the narrative voice throughout the story. They will jump from first person to third and back for no reason other than that they forgot which voice they were using between writing sessions.
Does nobody ever go back and re-read their own work?
Stylistic pitfalls are a messier subject, because that starts to tread into the territory of, "This is just how I write." While there is nothing technically wrong with some of the styles, I have noticed that they are often popular with novice writers. I recognize some of them from my early writing, and I owe a debt to a friend who helped break me of some of the habits.
One that I see often is staccato writing. The author uses really short sentences. The sentences are all grammatically correct. They are all strung together into a story. The story will have a character. He walks to the table. He picks up a book. He reads the book. He puts it back. He walks to the door. He opens the door. He goes outside.
I think you can probably see the problem. The story never gets a chance to develop much of a flow, and it becomes fatiguing to read.
Another style that I see fairly regularly is what I call the witness testimony style of writing. This happened, and then that happened, and then they went over there, and then that happened, and then he said this, and then he did that, and then... I started counting the uses of "then" in one story, and hit fifteen by the end of the second paragraph. As with my first example, it is not technically wrong, but it flows badly, and is very dry to read.
Two more styles that I often see with beginning writers are where every line is a combination of dialogue and action. Usually the story alternates between each of the characters in the scene, with each one taking a turn to say something, and then do something. The other style is what I call the Superman narrative. Virtually the whole story is told through narration by the characters. It's a style better suited to old radio plays than a written story.
"Why are you walking over to that table and grabbing the gun?" said the professor.
"I plan to shoot you, of course. See? Look at how I am pointing it at you and pulling back the hammer," said the mobster.
"Are you mad? Can't you see that I am just sitting here at my desk writing down formulas and smoking a cigar? Clearly I post no threat to you," said the professor.
"And now you pose even less of a threat as I have unloaded three bullets into you," said the mobster.
Finally, there are the writers who combine many of the above with a need to find their own voice by playing with conventional style.
"I am going to write my entire story in future perfect tense!"
The problem with picking a weird tense (even present), is that writing in past tense just comes naturally, and the writers invariably slip in and out of past tense as they are writing. My advice to them is usually to try and master the easy stuff before they start trying to stretch their skills.
I think the worst are the ones who decide they are going to write in second person. I don't know why anybody would write in second person other than when they are writing an instruction manual, or a "build your own adventure" story. Yet in my experience in some writing circles, this is a strangely popular thing among younger writers, and goes over remarkably well with some of the younger readers. I do not find second-person stories to be the least bit immersive, and in fact they often come across to me as slightly insulting. I don't appreciate a story that tries to tell me what I am doing, or thinking, or feeling.
My response to the author boils down to, "You seem to think you know me, but you don't. Please stop writing with the misguided conceit that you do."