- Sincerity (do you truly believe what you're writing?)
- Coherence (have you thought hard and thought it through?)
- Consistency, or Sauce for the Goose (does this opinion fit with your opinions on other matters? are you prepared to apply the same logic in circumstances where it leads to a less agreeable conclusion?)
- Skepticism (have you considered the counterarguments?)
Those are Michael Kinsley's words, not mine, and they come from a memo on how to write a good editorial he sent to LA Times editorial writers in 2004.
The memo is included in his latest collection of opinion journalism called Please Don't Remain Calm: Provocations and Commentaries. I just read it. Some of his essays are dated but most still resonate, all are under three pages long, and all display the wit that makes Kinsley required reading regardless of topic.