Blocking content won't protect children or prevent employees from bringing inappropriate content into the workspace. Those that want it will find a way to get it. If they can't get it on their computer, it will be on their iPhone.
The only solution to these "problems" is to educate your kids or employees as to why the content isn't appropriate.
It'd be an almost perfect way to make the cyber-nanny types shut up: all the naughty bits of the Internet could be tagged, and a client-side solution would be trivial for parents, libraries, etc.
Additionally, employers could save themselves quite a few sexual harassment problems.
And frankly, the NSFW tag could be argued for both sides of the content vs presentation debate.