I've become a bit of a prose connoisseur in my lifetime. For some reason, when I read, I don't look so much at the story (though of course that's important) as at the words that are telling it. Good prose for me is not a bonus, it's a necessity. And this doesn't mean 'grammatically correct' prose. That should be a given, with one exception-the occasional typo, though it can be annoying and confusing, I consider a credit to the author—it shows their mind is moving faster than their fingers can type. Stand proud, O ye of the many typos (then go and correct them, please—they're making it hard to read)!
That said, I break many of these rules (actually strident guidelines, see disclaimer below) in my own writing for various reasons, so don't go to me for examples of the good stuff. Although I do try.
DISCLAIMER: In writing there are no rules, but there are some guidelines that it is STRONGLY ADVISED you follow. And if you choose to go against those guidelines, you should have a reason. Partially because I enjoy brainstorming, and partly to help clarify my personal position on the following aspects of prose, I've added a few examples of times when I think you could violate those principles. I also give examples of what happens when they're violated for no good reason.
With that said, I believe good prose is…
Clear. If you can only be one of these things in your writing, be clear. Nothing causes as many problems as miscommunication. Say what you mean and say it baldly, avoiding phrases that might be confusing, avoiding paradoxes, and being sure the important stuff is said in a way the reader will remember.
You've seen this violated when: You read that story by the writer who fell in love with the words and forgot what they meant. When sentences ran on for half a page. When words are used in ways you are sure they weren't intended to be. When fragments show up that might be part of the characters' thought processes or typos—you can't be sure. When paragraphs of information are tossed at you in hopes you'll make sense of it, or when you're constantly in the dark because the writer won't tell you anything. When you had to ask questions every paragraph or so to be sure you're actually reading what you're reading. If you were really unlucky, the story was already published and you had nowhere to go for answers.
However, somebody might violate it because: The words are seriously more important than the meaning. Or the meaning is vague and each reader should be able to interpret it for herself. Sometimes muddled prose can also reveal the muddled thought processes of a character, or a character is schizotypal and reveals that through a first-person narrative. This might show up accidentally, however, in an effort by the writer to be fresh.
Organized. There is one topic per paragraph, and paragraphs come in an order the reader can understand. The story slides smoothly from one idea to another (I say story, but this applies to everything from personal essays to medical advertisements—wouldn't it be confusing if they said 'If you have any of these symptoms, see a doctor' when the symptoms were listed thirty seconds ago?). New concepts are not 'sandwiched' at the point the author thinks of them—right in the middle of an old concept. The reader knows where they are and it's clear how they got there.
You've seen this violated when: The writer didn't really know where he was going, so he kind of went everywhere. He had an idea that didn't seem to fit anywhere in particular, so he shoved it in at a point where it sort of maybe fit. And he forgot to reread before posting the story, so he didn't catch the problem.
However, somebody might violate it because: This is rampant in free-write exercises (the kind that don't let you self-edit) for obvious reasons. It can also be used for effect, to show a character overburdened with concerns and hopping from one to the other fretfully. Even so, some sort of meta-organization should set off these jarring tone and topic switches.
Fitting with the tone and setting. First, this means there are no verbal anachronisms for a historical or fantastic setting. The word 'okay' always jars me when said by a medieval knight. And before comparing the alien spacecraft to a saucer, check to see if your caveman protagonist has ever seen a saucer.
Second, this means that if the story is about a heavy topic, unintentionally light words are not used to describe it. For that matter, it should be obvious that the writer took care to choose just the right words to describe their scene of despair or horror. "Severed limbs were scattered about at random", an example I used in '15 Prose Mistakes Amateurs Make', violates this because the 'at random' (suggesting that this is opposed to methodically scattering limbs) draws the reader's attention away from the grisly horror of severed body parts.
Avoid 'bathos'—a sudden change in emotional tone. The example the invaluable writing resource "Turkey City Lexicon" gives is something like: 'There will be panic and riots in the streets, and chaos will rein unless the government starts being lots nicer about stuff.' It surprises the reader, makes them laugh, and makes you hard to take seriously. If you are not writing comedy, this can be an undesirable outcome.
The words used to describe a scene should fit like a soundtrack. With that in mind, you can break any of these other rules, and also the rules of grammar. Sentence fragments? Why not? Passive tense? Sometimes passive tense will be used (a protagonist written largely in passive tense will be looked upon by the reader as passive, which will work well if that's the idea you want to give). Clarity? Shmarity. Readers can fill in the blanks as they go—if they really want to.
You've seen this violated when: The writer uses whatever words comes to mind, creating a jigsaw puzzle of prose. You find yourself wondering what awkward phrasing they'll invent next just to get the words down on the page. It makes you want to rip your eyes out, or maybe pull the author's hair.
However, somebody might violate it because: I can't think of a single good reason why you would want to violate the integrity of your story. You might say 'but the battle will be described in funny terms because the character is a chaotic joker'. Fine, but that very lightness of tone is used because it is fitting with the character—therefore it isn't violating this principle.
Not overcrowded. Every word is necessary for either clarity or rhythm or both; each word should add something and that something should be worth adding. Wordy phrases are eliminated. The writer is not redundant and does not state the obvious if it is already shown through the action of the story (unless that would be fitting with the tone).
You've seen this violated when: People are wordy. People are verbose. They repeat themselves, or use words like 'that' which aren't necessary, or tell you things you already know. If you're editing them, you tend to strikethrough a lot of the text because it simply isn't doing anything.
However, somebody might violate it because: It's from the POV of a very verbose character. Or they're adding words to keep time to some sort of rhythm in the prose, perhaps iambic pentameter (they have my sympathies). Or they have a minimum word requirement.
Evocative. The reader should feel as if she is in your story, helped along by details in all five senses. She should have the feeling that there's a world beyond the story. Though many people will tell you that you shouldn't add details that don't move the story forward, you'll find that a quick comment about someone or something that doesn't serve the characters at this point in time will reveal that there's more to the setting you've created than meets the eye. Kate Elliot is a fantasy author who is very good at that—you see that she doesn't just want to tell the story and be done with it, but also show you around, give a tour, if you will. She never sidetracks the main plot, but a detail here or there about a beggar family or a man in the army planning to be married tells a lot.
You've seen this violated when: You're being told the story instead of shown it. More than that, you're being told the story as if it were a painful duty, best got aside as quickly as possible. There's no elaboration on anything at all. It's a summary of things that happened rather than a celebration of them.
However, somebody might violate it because: The writer isn't interesting in the goings-on at this point and is trying to gloss over them as quickly as possible. It's a short story or novella where every word counts and you're reading a part where detail isn't worth as much. The author would rather be subtle and hint at things than spell them out for you—but in that case, the prose will need to still evoke an air, probably of mystery.
Fresh. The writer does not rely on clichés to tell the story. Phrases like 'easy as taking candy from a baby,' 'cold as ice,' 'black as a raven's wing,' and 'this isn't rocket science,' don't tell the reader much anymore. Save your words and simply say 'it was easy,' 'it was cold,' 'it was black,' 'it was simple,' because that's all the image your reader will get anyway and at least now you can save a few words.
You've seen this violated when: It's pretty obvious. You see a lot of verbal clichés. This isn't rocket science, people.
However, somebody might violate it because: They're trying to win an award for most cliché story ever and plot triteness just isn't cutting it. Or they're going from the viewpoint of a character who uses clichés as a nervous tic. A cliché just seems to fit for some reason. Or they're going to use a cliché and then turn it on its head—'It's as easy as stealing candy from a baby.' 'You tried that once. You failed.'
So, to summarize that: be clear, keep your ideas in order, beware bathos and anachronisms, use evocative details, make every word count, and don't use a cliche to say something. I'm not guaranteeing you'll make a best-seller with this, but you'll at least avoid many pitfalls of writing. And your reader will appreciate having, if not a mind-blowingly wonderful, at least a painless read.
Update 3/2014: A lot of the information from this essay-plus a whole lot more-has now been published in my book, The Starter Guide for Professional Writers. I'll keep my plug short, but if you want to find out more about the book, which is jam-packed with information to help new writers with improving and publishing their work, please do check out my profile!