This list, of course, is tongue-in-cheek. Mostly.
1. Mystery fiction is not about character, society, politics, gender, religion or whatever else. It's about mystery and the puzzle plot is what makes its specificity as a genre. You will thus approach it with respect and attention rather than just as a template for your personal concerns.
2. Mystery fiction, however, is or should be more than just a game. Good writing, sound characterization and some humor can't but enhance the value of your work. Remember, though, that good writing doesn't equate preciosity or free-wheeling virtuosity, while sound characterization doesn't mean loading your characters with various problems and issues or going into painful detail of their childhoods and food habits, especially when neither are relevant to the story.
3. You won't conceal any vital clue or decisive element from the reader. This is an old rule, present in many past and present syllabuses, but it's always worth-repeating if only because it's infringed almost every day by legions of hacks as well as some major names. Cheating is wrong because it ruins the architecture of the plot, much like bad rhyme or meter errors ruin that of a poem; it's bad technique and thus bad art.
4. If realism and verisimilitude get in the way of a good idea then realism and verisimilitude should be dropped, period. One of the great pleasures of being a writer is that you can play God and do exactly as you please, so enjoy your freedom and let your imagination be your guide. Mystery fiction, unlike common misperception, don't have a sacred duty to "show the world as it really is". You are a writer, possibly an artist, not a reporter or a social scientist.
5. Conversely, do research only if it's your thing or if you think it may improve your story. Otherwise just go ahead and don't be afraid to talk about things you have little or no knowledge of - provided of course that you admit it from the start. It's fiction and most importantly, your fiction. The only master aboard is you.
6. Ellipsis is a force. Don't explain, don't tell, don't describe more than what's absolutely necessary and let the rest open for imagination and interpretation. Reading, especially reading mysteries, must not be a passive experience. Gaps must be left for the reader to fill.
7. Don't stay content with what others have said and done before. Try to find new ways, new themes, new approaches. Mystery may look quite strait-laced at first sight but it actually offers a plenty of room for experiment, and it would be a pity not to use it.
8. Series detectives are to be avoided as much as possible or used homeopathically, and must never become the sole or major justification for their stories.
9. Don't mistake gimmicks for originality. It takes much more than an "original" character, premise or setting to write a genuinely original mystery.
10. The genre didn't start with you and the authors that you like. To have a good knowledge of its history will help you understand it better and avoid hubris. Whatever "new" ground you think you're breaking, chances are someone else broke it a long, long time ago.