Before you can be published, you must get out of the slush and into the manuscripts that will command careful reading. Nearly every author's career necessarily begins with the slush pile -- the unsolicited manuscript. And there is so much slush that, in self-defense, professional readers -- editors, agents, even workshop leaders -- can make very quick judgments. Worse, because so many amateur writers make the same mistakes, professional readers become expert at seeing the fatal signs that, as early as the first page, even the first paragraph, can signal an author who in all probability is not going to be worth reading closely.
Avoiding these opening traps1 even for just the first page -- especially just for the first page -- is what it takes to move from slush to possible-read, the essential first step in your writing career. After that, it's up to your prose to sell your story, but at least you will not start with two strikes against you.
To move your manuscript from the slush-dismiss to slush-read piles, avoid these common traps (in alphabetical order):
Absence of tactile imagery. Forgetting to lock us into very specific images -- of people, geography, place, culture, technology, anything that makes this place and time unique.
Borrowed used furniture. Any story whose principal early elements are borrowed from other fiction invites the inference the author has no ideas of his own. When the orcs, trolls, elves and hobbits march in, readers march out.
Capitalizing Ordinary Words. Any time the hero contends with the Pit Of Slime, we are in trouble -- the author is using punctuation to substitute for invention.
Concatenation neologisms. New words invented by gluing together two common words (one often with an emotional connotation) to create the quick aura of futurism: devilcar, Trueflame, lightsaber.
Disengaged protagonists (often with superman syndrome). Characters early in stories often experience terrible events but are unmoved by them. If your protagonist is distant, so will the reader be.
Exotic spelling or unpronounceable names. Like italicized prolog monolog, it connotes fantasy-land without being specific, so it leaves us unsatisfied. In Peanuts, Linus once commented that when reading Russian novels, if he hit a long name, he'd just bleep past it. Readers do this. It disengages them from the characters. Be very judicious in using such names, especially first names. It's no coincidence that Frank Herbert's hero is Paul, Asimov's Harry (all right, Hari).
Forsoothly writing. Using florid phrases for ordinary expressions. "I circled my steed into position." In reality, no matter how grandiloquent it may sound to our ears, people don't talk like that, even in the past, even in a land of fantasy, especially when they are in normal conversation. Dialect and distant time are conveyed subtly, through careful choice of small words and short sentences.
Generic-land. Any standard-issue central-casting place is an immediate indicator that the author lacked either the imagination to conjure up somewhere new or the energy to do so.
Glacial startup. Authors, especially of first novels, who give us extended description or background of the world, the characters, their mechanics, with little or no action until 7,500 or more words into the narrative. Within the first thousand words, either give us the genuine out-of-whack event or a reasonable facsimile thereof.
Internal-monolog reminiscence. Classically expressed by a protagonist standing before a mirror, simply a device to dump enormous amounts of background, usually about the protagonist.
Organ music. Announcing, even before the protagonist has had a chance to establish himself, that Momentous Consequences Tremble in the Balance. Stories should never announce their importance; rather they should engage us with the characters at the characters' own level, and move into significance only as the stakes rise.
Portentous prolog. Almost invariably, they are info dumps that mean little to the reader but instead slow the story by creating Heavy Overtones. Often they introduce a significant if distant character (an alien, a mage, a nefarious antagonist) whom we will later encounter. Usually written into the story because either (a) the author is working out why an event occurs, and having conceived the idea, cannot resist putting it onstage, or (b) the author is not confident that the reader will follow an ordinary introduction unless you tell him that Huge Stakes Are Involved.)
Proper-name overload. Excessive early use of proper names suggests a frenetic narrative that discourages readers.
Quoting from imaginary texts. Because Herbert did it in Dune, everyone thinks this is a neat way to bring in the exotic world. All too often, it is merely portentous prolog in verse.
Telling too much too soon. Authors new to their own imagined world tend to want to give readers a guided tour even to the extent of having characters tell one another things both would either know or not care about. Imagine a story set in Omaha that took time out to say, "two hundred years earlier, President Jefferson had authorized purchase from the Emperor Napoleon of the Louisiana Territory, which was later explored by Lewis and Clark." Don't tell us anything that the characters don't need to know, and only when they need to know it. This forces the author to be clever about sneaking in the info. It also moves the plot and makes the reader work ... and a reader who is working is by definition paying attention.