1. A good mystery writer thinks first and foremost about the reader's experience.
2. A good mystery writer balances character and plot.
3. A good mystery writer thinks about the future.
4. A good mystery writer listens to and synthesizes the advice of agents, editors, and readers
5. A good mystery writer takes him/herself seriously, but not too seriously.
6. A good mystery writer pushes and challenges him/herself.
7. A good mystery writer understands the competitive landscape
The annotations provided on the blog flesh out the skeletal list with meaty tidbits, which you can read more about here, and are still as true today in the new publishing landscape as they were then.
On the more creative end of things, the Guide to Literary Agents blog once listed The Top 10 Reasons agents stop reading a manuscript and/or throw it across the room (this time in reverse order):
9. Spoon-feeding the reader what the character is thinking
8. Having the characters address each other repeatedly by name, as in, "John, let’s go!"
7. Introducing a character with first and last name, as in, “John Smith entered the room.”
6. Beginning a story with dialogue
5. Opening with a cliché
4. Yanking the reader out of the action with backstory
3. Not giving the reader a sense of place or where the story is going
2. Characters are MIA until bottom of the second page
1. Telling instead of showing