What is good and bad about free speech?

Bad: most people who use the term "free speech" don't understand what it means and what it doesn't mean. Have a look at this handy xkcd.

Agreed. I am in the process of creating a lemmy instance (mostly for testing), with the core tenets being free speech and freedom of information. I just need to find a way of rewording it so that people don't think it means endorsement of assholery.
I think invite-only and "Don't make me ban you!" as the only rule could work.
- Some inspiration for what to include/how to phrase the rules:
  "Be nice to each other"
  "You are allowed to share your opinion. Others are not required to like it"
  "Accept that others might have different opinions from yours, just as you would want them to accept that your opinions are different from theirs"
  "Moderation is based on how you say things, not what you say"
  "Free speech has legal limits in most jurisdictions. The instance owner may be forced to remove illegal content even if they agree with you." (for example, saying that all billionaires should be killed may or may not be a valid opinion but it may be considered incitement and depending on where you live, instance admins can get in trouble for not deleting it)
- "Don't make me ban you!" Isn't a rule.
  A rule would be "don't say anything I dislike"
If you intend to wax pedantic then at least give us your definition. This secondhand linked pedantry crosses the line.
- Free speech means that you can not be punished by law for your opinions. It explicitly does not mean that others are required to listen to you or even like your opinions. Just as you are allowed to hold a controversial opinion, they are allowed to disagree with you, argue with you, walk away or show you the door if you're in their house/community/instance.

Good: I have free speech.

Bad: You have free speech.

I think that sums it up.

"Free speech" is neither good nor bad. Rather, it doesn't exist. Speech, ie what is discussed in the court of public opinion, is dominated by whatever mechanisms and institutions that have power. In countries like the US, this means large megacorps and billionaires can flood the information space with articles flattering their positions. Companies like google can censor importabt but inconvenient information from searches, and more.

Speech is controlled by whichever class has control, be it bourgeois or proletarian, or even aristocrat.

But I think you’re wrong ;) megacorps share my thoughts and feelings

It prevents others (the government, moderators, social media platforms, parents) controlling the narrative and allows challenging those in authority. It enables minority opinions to be put out and considered by the majority.

Any detriments are mainly 'people are rude', but that's not reason enough to outlaw free speech, given the costs of doing so

None of that's true. Free speech laws try to prevent the government from arresting you for opinions or criticism. Social media platforms, parents, etc are still able to take action against statements without reason. The government can also put the blame on something else. If someone is critical of the government, they're likely to have broken laws they don't agree with.

There is no inherent arbiter of good or truth.

If the trust in expertise is tarnished, reshaping reality is left to the uninformed.

You can trust the uninformed to express their own will truthfully at least. (Unless they're convinced to side with an authority). There's a reliable truth there. I guess that's the rock of democracy.

"Good" and "bad" are pretty hard to define.

This question reminds me of the "Why is the Ameican government the best government in the world"-scene in "Thank you for smoking". (Recommend the movie, not the American government)

Good : It makes the conversation intelligent and sane.

Bad : People sometimes say bad things.

Free speech in no way makes a conversation intelligent or sane.
Free speech is necessary to prevent the government from censoring dissent.
The consequence is that there's little to no legal repercussions to spreading lies and hate. But that doesn't mean there can't be repercussions.
- Imagine a censor who thinks that all of his weird dogmatic opinions are pure golden truth picking over your conversations and rewriting or removing everything he doesn't like. A rather stupid, sloppy censor who couldn't get the point if it was underlined twice.
  Imagine what your posts would convey then.

You can say what you want except when you get arrested for it because free speech is fake.