One of the most frustrating and overlooked problems that I see in fanfic (or original fiction) is poor placement. There are a few guidelines about this that I feel should be noted.
To begin, objects should not pop in and out. They are not allowed to suddenly exist just at the moment when they are needed, with no prior reference. Similarly, they should not just disappear when they are no longer in use. Objects of Special Importance in your fic should be discussed well before they are used to destroy Evil Overlord, Mr. Deleterious, or before they unlock the Secret Chamber of Doom. Things like this should be introduced to your reader prior to their shining moment so that the reader isn't confused by the sudden existence of such an item. Not to mention the fact that it makes your author look lazy, as if they are just taking the easy way out because they didn't think about their plot beforehand. (Your author may, in actuality, BE lazy, but it is unnecessary to call attention to it in this way.)
Characters also may not play jack-in-the-box. If they are in
This also applies to smaller movements. One cannot simply have two pages of dialog and no action. I like to refer to this phenomenon as the “talking head” syndrome. It is necessary to track the movements of any given character throughout their environment. My rule is this: if, when reading through a scene and specifically looking for character movement, I have no clue what a character is likely to be doing while they are talking, there needs to be more explanation. This is not to say that the author must tell of every possible action the character takes. Rather, it is more effective to hint at their movements and let the reader fill in some of the blanks. But the hints have to be there in order for that to be possible. Particular attention should be paid to the objects that the characters interact with, including food, seating, clothing, anything they are holding, etc. This is an easy way to work in action in a subtle manner. Take note of their actions and the objects around them. Is George scratching his nose, strumming a guitar, walking up the stairs?
On a related note, an important thing to avoid is beginning a conversation in one setting and finishing it in another, with no real explanation in between. This causes the characters to magically move from the cafeteria to their home, simply by talking. NOT ALLOWED. Your author MUST keep track of these things. Do your very best to point out such instances to your author, and help them to keep their characters "grounded" in the fantasy of their choice.
Also remember your timelines. If it is winter, there can be snow on the ground, it will (probably) be cold. Trees may be bare, people are wearing sweaters, scarves, hats, coats, etc. Sickness goes around, people are snuffling from the cold. What about frostbite? Also remember that winter eventually turns into spring. If the fic goes a whole year in nothing but winter, there'd better be a reason for it other than author oversight. Also keep track of night and day, east and west, altitude, etc. A character facing east in the evening should not have the sun in his/her eyes.
A good way check for these problems is to do a walk-through of the fic you are editing, looking specifically for this sort of thing. Do your best to place the characters in your mind and really see the scene. If you get confused, let your author know. Take note of new objects and make sure the author follows through with them. Don't be an object nazi, but make sure your author is clear, especially in action scenes such as car crashes or fist fights.
As in all cases, these are merely suggestions. There are no real hard and fast rules to what is “right” or “wrong” in literature. Any rule that has been made has also been broken with fantastic flair by someone, so use your best judgment.