The first is detail consistency. Some of this seems pretty straight forward. If your character goes to the gym on Tuesdays and Thursdays, then you can't give her a gym scene on Friday without explaining why she's there on her off day. Makes sense right?
So why are so many authors leaving out the why?
Here's an example, in an unnamed book I am reading the opening scene is a doctor trying to stitch up a man's ankle in the emergency room. This is made difficult because a storm has knocked out power and there are no lights (yes, I know there would be a back-up generator at a hospital, but apparently the author does not). The author spends a lot (a lot) of time describing the difficulty of putting in these stitches using only the occasional lightning strike to see. We have a crystal clear picture. And then...
Another critical patient comes in and we see a whirl of activity while doctors and nurses hook up central lines, examine wounds, apply bandages and use the shock cart. During this entire scene no one has any trouble seeing anything.
Now maybe this patient was taken to an area of the hospital where the lights were on. Or maybe they were having a hard time seeing. The point is, I don't know, because the author didn't tell us. All the sudden, this detail that was hammered in our head for fifteen pages is just gone. The glue holding the scene together has dissolved.
The second area I want to touch on is character consistency. This is a biggie for me. Failure to keep your characters consistent is an epic fail. I'm not saying your character's can't grow, but I need to see that growth, not assume it.
Here's another nameless example to bring home the point. Character A has been ordered by his boss to get a valuable object from character B. Here's what we know about character A: he is shy, always follows the rules (due to his personality and his cultural upbringing), is excessively polite, does not have any home invasion skills (like lock picking) and, most importantly is afraid of character B. Character B is a highly armed, very skilled professional body guard. So what does character A do? Obviously, he breaks into character B's apartment and steals the object. What? No, why would he do that? The character A we've come to know would simple ask character B for the object, using please as many times as possible.
In this case, the author made a character do something because the plot line called for it, not because it was a probable action for the character. This is a cardinal rule, character trumps plot every time. There are few rules in writing, but this is one of them. If your character is going to do something out of character there must be adequate motivation.
Failure to be consistent in your writing is like taking the binding off a book. Pages start falling apart and very shortly you'll find yourself with little more than a hot mess. Or a can of beans. Your choice.