Re: how about a simpler system
That was actually considered maybe twelve years ago, with .xxx. It got very political, in the US, with a lot of concerns that ICANN was being pressured by the Department of Commerce on behalf of the federal government. It didn't succeed.
The porn industry opposed it: They were concerned it would create an 'internet ghetto' where their content would be walled off and widely blocked by default.
The religious right, who were very powerful back then (This being during Bush II's term), opposed it - because they thought having a government-endorsed place on the internet for porn would legitimize it. They wanted to ban it completely, not create a new place specially for it. Also they were concerned that even if .xxx were made, sites would have no reason to close their old domains down and lose the established traffic.
The only group that actually supported it was the registrar who was spending a lot of money lobbying for the approval of .xxx, as they hoped to make a great deal more selling domains.