What about:
4) Human moderated in smaller communities
5) Self run reputation systems
Plenty of communities exist out there that aren't a free for all, do not perform id checks, have adult content, and are free from children.
> 4) Human moderated in smaller communities
People have basically decided that they don't actually want this.
I know I don't want to visit 10+ different websites on regular basis to keep up on discussions of various interests I have.
> 5) Self run reputation systems
Isn't that basically the upvote/downvote systems used by reddit and HN?
Neither of those address the problem of children accessing adult content. If there is no ID check how do you know there are no children on them?