Creating new
top level domains which are restricted by
content or
ownership. Existing ones are
.aero,
.museum,
.coop, and
.pro. Proposals such as
.sex and
.xxx have been kicking around forever, and
.kids.us looks like it might become law.
The basic problem here is that there is no reason to expect a name to take on this role. Book titles are not expected to always be honest, and nobody is yet regulating library card catalog numbers. Labelling content can be much more productively done with an in-page markup like PICS--- although hopefully the markup would allow authenticated rating systems from a variety of third parties. (Perhaps there's an end to end model argument that content restrictions belong at the content layer, not the naming system?)
The structure of the Domain Name System currently makes it necessary for one organization to decide who is in and who is out of such domains. In addition to the economic windfall if the domain is mandatory, the power granted to the controlling organization is directly proportional to the usefulness and popularity of the domain. Do you want to trust some single body to decide who is a union, who is a professional, and who is a school for the entire planet?
In the meantime, lots of people are making good use of domain names for expressive purposes, excercising their free speech rights without any respect for the artificial divisions of the namespace into ccTLDs and gTLDs. goatse.cx is one familiar to Slashdot readers, but .tv and .to allow lots of cute examples. And the commercial world seems to have accepted the fact that we have a flat namespace after all.