Why are some missions "Good"?...

[Flyby]

to me a COOP good mission has enough immersion to keep a pilot busy and a successful outcome is not a sure thing. It need not be historical, but should provide a high "pucker" factor.
A bad mission is just the opposite: too easily accomplished goals, and the outcome is never in doubt.

I've produced some so-called Die-by missions where survival is always in doubt. Maybe the plane-sets are unequal in the opposition's favor. Or air starts with the enemy coming in out of the sun. Or very well defended ground targets. As I said, I prefer a high pucker factor.
Flyby (Die-by missions specialist)