The worst habit, that you will encounter very commonly on any fiction-writing community open to everyone is that many people write scenarios rather than stories.  They tell a fictional anecdote, but it lacks a conflict.  For example:

Once there was a group of people who wanted to go to overthrow a dictitorial government.  So they created an army and overthrew the government.

As you can see, the story lacks conflict.  This is a very simple example, but I have read some stories – normally a bit longer, but just as poor.  That’s a habit that far too many people have.  I’m not normally one for discouragement, but if you write like this, then you have no talent, you’re a wannabe, and you might as well give up.

Another bad habit I see is lack of details.  Storytelling is not just telling stories, it’s an art.  You can’t be interesting by simply explaining a storyline to people, you have to paint a vivid picture in their mind.  If you just tell a story, you’ll be boring.  You need detail.  Compare these:

He went to the building to arrest the criminal known as Crazy Dave.  As he walked in, he saw the Dave.  After a fight in which he was wounded, he arrested the the criminal.

He walked up to the elegant mansion to see if he could solve the case involving the notorious gangster known only as Crazy Dave.  He walked through the mahogany doorway to see Dave sitting on the couch as if he were expecting an officer.  Two thugs jumped from the ceiling and attacked the officer.  The fight with the two bulky criminals proved to be a tough one.  One of them swung his knife and cut the officer’s arm.  Enduring the tremendous pain, the officer managed to defeat the two thugs and arrest Crazy Dave.

As you can see, the top one can put you to sleep while the bottom one give you the ability to visualize the scene as it happens.

The final bad habit I see is one that really bores people, especially for longer stories, such as novels.  The problem is that the storyline becomes fixed at the beginning of the story.  When this happens, the storyline never changes.  When this happens, no matter how talented the writer is, the story will get boring after a while.  So ensure that your story has its twists and turns, or people will fall asleep on you.

Storytelling is like any other art.  It takes talent, and more importantly, technique.  If your stories aren’t descriptive or they lack any storyline, people just won’t read them.