All sites should oppress hate speech.

Hate speech and threats of violence should be a no go but that's really it imo

That's a good point. Why create a site to allow people to speak if you are going to tell them they can't say this or that?
I guess the answer to your question cannot be formulated cause everyone has a different opinion as to what can be said and how much free speech should be allowed.
In my opinion there should be 100% free speech. I don't understand the desire of anyone to control or censor someone else based on their opinions. You could tell me the worst thing in the world, I would totally counter argue and deconstruct your logic or opinion if I disagree with it but I would never censor it, it's just not who I am.

However a forum with no mods where everyone can say everything and anything will quickly degenerate into insults, attacks and such a forum would quickly become impossible to be a part of cause people are incapable of behaving on their own, just like the illusion of safety when you go out in the street. The ONLY reason you don't get systematically killed or mugged is because there are laws that keep malevolent people at bay. And even with laws you still have lots of violence and danger out there...

So a forum needs to have rules and mods and all that stuff. And that's when the opposite problem happens, rules that are too strict or unfair or biased in one way or another, or mods that go on a heavy handed power trip. This varies from forum to forum and I have never met a forum that has struck a perfect balance.

I see no solution to this, as long as forums or societies or countries are ruled by humans that are by nature flawed, there will never be a perfect balance of rule vs freedom. We as humans lack the wisdom needed to achieve such high level of enlightenment.

In my opinion a perfect society, country or forum is one where there are absolutely NO rules and it would be up to everyone to be benevolent and honest enough to simply cause no issues and to the people you talk to, the same level of wisdom and benevolence to respect an opinion that may be totally different from yours and that you'd respect and argue kindly without causing any problems in return. But that is a utopian vision that I have and nothing more.

And who decides what constitutes hate speech? You? Me? The site?

See, it's not that simple I'm afraid.

The site obviously, since they're the ones with the ability to do it.

Yes within general morals and values

I mean who decides from a moral point of view.

You are talking about the "power" point of view. The site, the government etc... cause they can, they have the power. But that's not the issue here, the issue is who is right about their decision that something is right or wrong.

That's up to the admin and creator really. Though there shouldn't be a state judging whether or not something someone says on a site should be legal or illegal.

The site should allow or not allow whatever it is they want. And people can choose to use or not use the site based on this. Free market baby.

The site/moderators. Sure, they can never be completely objective, but there are a lot of things that clearly shouldn't be allowed. There are laws against hate, and sites shouldn't let their users break these laws.