No Rules

There are no rules in writing. There are only statistically relevant guidelines.

Writing is entirely subjective; there is no absolute good and bad, no absolute right or wrong, no absolute "you must do it this way." No matter how you choose to write, you can always find someone who will like it (within reason; 200,000 misspelled words in a row will probably not find any takers). And if your goal is only to make one person happy, then what does it matter whether you follow some arbitrary "rules"?

But most of us have bigger goals: We want to make enough money writing that it can be our career, and in order for that to happen, enough people have to like what we write. The more people you want to please with your writing, the more statistically significant those guidelines become. Most readers expect certain things, to wit:

You really can't violate standard spelling. Despite the fact that English has no central defining authority, spelling has become almost entirely standardized, with only minor regional dialects.

Grammar is a little less fixed, as there are a number of edge cases where you can bend the rules without any significant number of people perceiving it as a mistake, but generally, you've got to stick with fairly standard grammar or you're going to confuse your readers.

Things get fuzzier as you get into plotting, pacing, characterization, character development, descriptive style, and all the other elements of the actual storytelling. There's a lot of flexibility in how you can do these things, because there are a lot of different readers out there who will tolerate variance. There are individual readers who will tolerate slow pacing and fast pacing, but maybe not in the same book, or simply not at the same time—Joe Reader might well be up for a fast-paced detective story today, but next week maybe he wants something a little more laid-back and reflective. A single book can't please both those aspects of Joe Reader's personality at the same time.

Rules about how to write are legion. Here's one which includes rules by (among other well-known authors) Elmore Leonard. Dare you, Jane Unpublished Author, think you know better than the great Elmore Leonard? The problem is that some of Leonard's rules are violated willy-nilly by other famous best-selling authors. Does that mean Leonard's rules are bullshit?

No; it means they're guidelines. All things considered, following Leonard's rules will probably produce prose prone to popularity... but not always. (And to some degree, his rules are stylistic choices. "Never use a verb other than 'said' to carry dialogue," he says. It's trivial to find bestsellers (and critically acclaimed novels) that violate that rule. Maybe he's just trying to reduce the incidence of using verbs other than "said," knowing that eliminating other speech verbs is impossible.)

Pick your goal. Figure out what guidelines you need to follow to meet that goal. Then get your ass in the chair and write, write, write. You don't always win when destiny rolls the dice, but the more times she rolls, the more chances you have to win.