Good points all around. I think what's realistic is defacto segregation, and in particular segregation based on voluntary association rather than race (as determined by a government official).

Just letting people create communities and decide amongst themselves who can join solves a hell of a lot of problems, and the problems it doesn't solve, it decentralizes - so those people can deal with them inside their own community.

As I've said before, I think there's a lot of potential in just making landlords insure their tenants against criminality, because the landlords will then make the tenants buy insurance, and people who are unruly will just not be able to afford the insurance to live in a polite neighborhood. Also this kind of thing has the potential to actually gain adoption, unlikes a lot of completely pie-in-the-sky stuff.