Just for the sake of discussion, what would you tell someone who wanted to be a stand up comedian or comedy writer about the qualities of a good joke?

has to be interesting. Too many details will boor people.
A good stand-up has a delivery that seems natural. Like he is talking to you about something he saw or did. Or giving his opinion on something. The more honest it seems, the funnier it is.Being very good with descriptions is great. It is easier to describe something by comparing it to something similar (that girl looks like a fish ) than trying to describe using adjectives. It’s faster and keeps the flow moving.
The above answer summed it up. If you want to know, go to a comedy club. When a bad comic goes on, you’ll see why he is bad. When a good comic goes on, well, even if he’s not funny, you’ll still want to hear the story. Even when the jokes aren’t that good, he manages to captivate the attention of the audience.