Sites that allow cross-posting usually have a problem with abuse -- pervasive cross posting is basically a form of spam. Is there an analogous problem with tagging? And if there is, what is the limit to the number of tags you can add to a thread.
That's where using aggregate info can help. And the tags rely on the wisdom of the crowd. So even if someone abuses it to give it irrelevant tags the more the people use it the less visible that abuse is.
So for a specific example, let's say someone tags this topic "ass". Only the top 5 or so tags are displayed for each topic, so all it takes is for 5 better tags to push that off the list. Eventually, we'll do things like make a minimum for it to even appear there (again, we need scale for something like this).
On the tag pages, the topics most tagged something appear more prominently. So again, the more it's used right the less prominent the abuse will be.
Although I like the ability to simulate cross-posting by combining tags, what I'm lacking at the moment is an intuition for new tags that are useful, but didn't exist as forums on the old site. Perhaps you can give us a few examples so we get the hang of new tags we may add?
Sure. A great example would be our science and mathematics forum. Right now everything imported from there is tagged "science and mathematics" but often it would more appropriately be "science" or "mathematics" or even "biology" and other examples of sub-topics to these more inclusive tags.
So for example, Sozobe's huge obama thread was imported tagged "politics" and I tagged it "obama" and if enough others do it will be at the top of the "obama" tag on this site as the most relevant obama topic.
So to use your own threads as examples, you have a thread about Iselin, New Jersey that is currently only tagged "north america". You can tag it more specifically to "new jersey" and better focus its categorization. There are a lot of ways you can give it more specific labels, just let me know if you want more examples of what I'd tag your topics.