That's an issue we were talking about when discussing a blockchain variant of throat. The downside to this is when you have illegal content and need it to be gone.
To me the biggest problem with things like this is it doesn't attract smart people. Someone puts a bunch of their time into making a cool thing, and then the userbase ends up being total garbage because anywhere that advertises itself as a place where you can post anything you like just attracts retards, which in turn causes posts and stories to pop up about the place which attracts even more of them.
In theory it would be nice if there could be a thing like this, but the reality is that in practice it just ends up turning to shit. Nobody creates a neat thing or comes up with consistently good post content and decides to share it on an obscure anonymous message board.
As opposed to the top minds on Facebook, Twitter, and Reddit? My experience is smaller sites and communities--particularly those that have a technical barrier to entry--attract those who at least have something interesting to say. Oh and trolls. There are always trolls, but you just got to grow a little thicker skin and try to look at the bright side of life.
Governments aint gonna like that