Post-Note: Life tag is abused in this case. just to be fair.

Tag curation would be a good solution to this. People disagree about the meanings of a tag (the sex tag is the perfect example, some people just want to talk about sex while others might think that explicit, pornographic pictures are appropriate). Semantic tag curation would mean that some users could more easily make a strong definition for what a specific tag means and what it doesn't, making it closer to the moderated subs on Reddit.