There's always debate about the "rules" of writing and how strictly they should be followed. I, for one, tend to be in the "Egh, if I feel like I'll do it" camp and I'm not much of a stickler for anything. (Well, except maybe Beginning, Middle, End and Outlines Are Not Your Enemy.)
But two things about these debates always strikes me.
1) Less experienced writers tend to assume they are more advanced than they actually are and proudly flout rules they would be better off following.
2) Less experienced writers tend to cling to the more absurd rules while ignoring the tried and true.
(Of course, on any given day I still fall into the "less experienced" category. That is not meant to be derogatory, simply a statement of fact. As I have gotten better at what I do, I have found more (not less) merit in some of the rules that I used to think were stupid.)
The big question, naturally, is what are these rules? Every writer probably has their own list of Writing Rules. Here's a look at some that I have run across.
1) Grammar/Spelling/Basic Sentence Construction
This is probably the ONLY rule I consider absolutely necessary. Nothing ruins a good story faster then rampant spelling errors, obvious misuse of punctuation and/or parts of speech, or sentences that leave the reader wondering whether it was the mirror that was frightened or the girl staring into it.
2) Never Start a Novel with a Prologue
There are many good reasons to not use a prologue. Take a serious look at them before deciding to go with that "introduction to the plot/MC/MV/world/whatever before the beginning of the story".
3) Never Start a Story with a Description of the Weather
Again. There are very good reasons to avoid this. Research what they are before you write that prologue to the epic fantasy that has a two page description of the storm cloud lingering over the Mountain of Death.
4) Never Start a Story with the MC Waking Up
Good reasons here too. They boil down to: most times this indicates a false start to the story and lazy writing.
5) Never Have the MC Look at Him/Herself in the Mirror in Order to Describe Them for the Reader
Yes. Lazy writing. And character appearance is rarely as important as you think.
6) Don't Write in First Person/Omni/Present Tense
This is mostly personal preference, but certain forms of writing are more difficult than others and should be approached with caution and a good bit of research into what makes them work and what doesn't.
7) All Chapters Must be a Certain Length
This one is just, well, stupid. But certain parties seem to think it's law. You should probably ignore them.
8) Never Start a Story with a Dream Sequence
This is actually a good one. Especially if the dream is one that results in deliberately misleading the reader. ("What? OMG! He's a serial killer? Oh. He just had too much pizza for dinner.")
9) Always/Never Give Your MC a Trendy/Cool/Symbolic Name
This one is stupid only because naming characters is hardly a thing of absolutes. HOWEVER, it isn't a bad idea to think twice about having a moody, dark-haired MC named Raven.
10) Always Start With Something Interesting
The main flaw with this rule is that it is frequently misunderstood. Interesting does NOT mean things have to explode/people have to die/MC discovers he's an alien/whatever on the first page. It DOES mean that however your story starts it should make us interested in the character and curious about their situation. Even if it's ABSOLUTELY ORDINARY.
There are, of course, more rules than this. These are the ones I run across most frequently. Some I keep and others I break. But that's a story for another day.