from an amateur fanfic writer.
1.) Don't put 'my summary sucks/I can't write summaries' in your summary. If you can't take the time to write a decent summary, what's gong to make the reader think you can take the time to write a decent fic?
2.) Don't beg for reviews, and for the love of God don't put 'I won't update again until I get X amount of reviews' at the end of your chapters. I see this all over the place and it makes you look just as conceited as you think it does.
3.) Get a beta-reader. I don't care who it is, your sister, brother, friend, mom, English teacher, whatever. Get someone to go through and read your shit. You'd be amazed what another set of eyes on your writing can pick out. For a lot of readers (including myself) all it takes is one big error or lack of punctuation to pull me out of your story and douse my interest. Most people aren't going to struggle through it. They're going to click 'close tab' and move on to the next piece of fic.
4.) Don't take your readers for granted. I always assume whoever is reading my story can read at an average level. That is, I don't have to spoon-feed them every bit of information. People want to read stories, not lessons. This includes, but is not limited to:
Telling us who's POV a certain part of a story is.
Any sort of character information sheet.
The time, date, or place. See #5.
Lists. Please don't prattle of every little thing that your character has in his inventory. I see this a LOT in L4D fic. Nobody wants to know the exact make and model of your character's gun. Most people aren't going to see the words 'Nick's Steyr SSG Elite Tactical Rifle, chambered in .308' and know what the gun looks like. Just describe what it would look like to someone who's never seen it. The Steyr is a big gray rifle with a scope. That's all you need to know.
Placing author's notes right in the middle of your prose. I can't believe this has to be said.
5.) Show, don't tell. (I myself have to work on this, but I've heard it so much that I figured I'd share it.)
6.) Constructive criticism is not trolling. Don't get your panties in a bunch because someone pointed out your errors. You should be happy someone took the time to tell you it was wrong. I know it's hard to take criticism, but it's sort of the way of life. I've talked to countless fanfic writers who apparently 'have offers to write professionally' and 'have gone to college for writing' and 'this person or that person thinks it's awesome'. Really? Because if you had any pride in your work, you'd want to fix it. You'd want it to be the best you can make it. If you can't even do that, you have no business writing anything-- not even fanfic.
7.) The best advice I ever got in regards to writing is to 'write the way you want to read,' and it works. If you start to get bored of writing your own story, chances are most people are going to get bored reading it.
8.) This one is strange, but stick with me. Read your dialogue out loud to yourself. If it sounds weird, change it.
9.) I know this one should be obvious, but you'd be surprised how many people don't use a spell-checker. Fanfiction.net even has one built into their document editing page. Use it.
10.) Don't use all-caps in lieu of italics.
11.) Read other people's fic, even if you don't like the pairing, or the subject material, or the genre. Feedback is an incredible tool that can make a terrible fic into something worthwhile and amazing.
That is... if the other 90% of writers accepted concrit.
I wanted to rant. Sorry. The L4D section of ff.n is a terrible cesspool and it amazes me that it's allowed to get this bad.
I'm not a professional writer, and I never will be, so I don't presume to know more than someone who is. This has sort of been brewing a long time. I'm getting sick of being called a troll when I (helpfully) point out errors. I'm getting tired of people not giving a shit about the stories they are posting. The written word is one of the most powerful tools we have as human beings, and a lot of these people-- these authors-- couldn't care less.